The heroes are fighting a near-immortal Big Bad and his second-in-command. Both of them appear to run on some sort of "life force" or have several lives... the only thing for the heroes to do is keep fighting to wear them down, hoping to get them down to nothing. Slowly, the villains realize the tables have turned, and they're losing...
So the Big Bad gives his life force a little boost by draining his companion to death.
Usually, the two villains get on very well and trust each other before this moment, just for that little extra Kick the Dog punch. A heroic variant involves a lancer or similar trusted person giving their energy to the hero in a Heroic Sacrifice. Being heroes, it's the devoured that makes the choice in such cases and the devourer is usually reluctant.
This is a sister trope to Villain Override. See also Just Desserts.
As this is a betrayal trope and frequently ends in death, beware of spoilers.
In Digimon Adventure, VenomVamdemon eats his subordinate PicoDevimon for a marginal powerup during his Chewing the Scenery sequence. And he doesn't stop there, either, ultimately absorbing all of his surviving minions.
Devimon also absorbed The Dragon Ogremon by turning him into Black Gears and absorbing him. In an unusual instance, Ogremon actually survived because he was blasted out of Devimon during the resulting fight.
Also, in Digimon Frontier, Lucemon does this to Crusadermon and Dynasmon shortly after their defeat by the Digidestined.
Happens all the time in Digimon Xros Wars due to the villains having the ability to Digi Xros with their underlings, whether the poor Digimon want to or not.
An interesting subversion is when DarkKnightmon tries to forcibly fuse his superieor, Bagramon into himself, in order to gain full power. He actually succeeds...for about one second before Bagramon reverses the Digi Xros from within, becoming the devour.
In Shinzo, the Mons that have replaced humanity turn into cards after being defeated, and they can power up by assimilating another Enterran's card. One villainess, a general in the Big Bad's army, killed five of her peers well before fighting the heroes and used them to achieve a One-Winged Angel form.
Fall absorbs Deuce's magical power (killing her) in her final fight in Nora: The Last Chronicle of Devildom. This holds something of an inverted Heroic Sacrifice because Deuce throws herself in front of Nora's spell to protect Fall, then apologizes for her weakness as Fall absorbs her afterwards.
Hiten eats his younger brother Manten (more specifically he eats the shards of the Shikon jewel implanted in his brother's head) after he's killed. Also a subversion, as he genuinely cares for his brother, and is using the strength to avenge him.
In an anime filler arc, the leader of a tribe of panther demons does this to some of his subordinates when he realizes he is losing. Made worse since his tribe spent most of the arc gathering the shards to revive him after Inuyasha's father killed him in the past. The panthers give up the fight after their leader is defeated realizing he wasn't worth avenging.
Szayel Aporro Granz from Bleach has specifically designed minions which he can eat to heal himself. In the manga, he actually eats the minion, but in the anime he turns it into a purple orb and then eats it.
Giselle of the Sternritter injected her blood into Bambi, converting her into a powerful zombie. After Giselle's near death she drank the blood out of a pleading Bambi to restore her own health.
When Yhwach infiltrates the Palace and his elite guard is killed he absorbs the energy of every Sternritter not directly assisting him and uses it to revive his guards.
In X/1999 the TV series, after Fuuma has had his arm and half his face blown off, he orders Nataku to liquify and became a human skin graft, restoring himself for the final battle at the cost of his last henchman's life.
Tengen Toppa Gurren Lagann does this in a very strange, but very awesome way in the final episode. Lordgenome, the reformedBig Bad of the previous arc, absorbs the Anti-Spiral's ultimate attack, converts it into energy, and allows the heroes to devour him for a power-up.
In The Stinger of Kill la Kill's episode 23, Big Bad Ragyo absorbs her secretary Rei Hououmaru (who until then had done almost nothing) into her body, to power her ultimate outfit. Hououmaru stoically resigns herself to her fate. When she is forcefully ejected from Ragyo in the next episode, Nui Harime feed herself to the Primordial Life Fiber, which then fuses with Ragyo.
Orochimaru intended to do this with Sasuke. It ended up going the other way around.
Also, Kabuto later integrates Orochimaru's remains into his body to make himself stronger. It was ambiguous for a while who had devoured who, but Kabuto seemed to be more-or-less in control. Orochimaru still came back, just via a different route. He then reabsorbed the bits of himself in Kabuto.
Possession of the Rinnegan falls under this. After Pain's death, Tobi killed Konan and stole the Rinnegan from Nagato's corpse in order to gain a massive power boost for the upcoming war. Much later Madara used Black Zetsu in order to take the Rinnegan from Obito for himself.
Mag Mel from Bakugankills and absorbs Mistress Sellon after she had outlived her usefulness, using her energy to power himself as he prepares to carry out his ultimate plan. The very next episode, he does the exact same thing to Anubias.
The Heroic version of this trope occurs twice in JoJo's Bizarre Adventure. The first is with Jonathan Joestar and his mentor Zepelli, who gives Jonathan all of his Hamon power shortly before his death. A similar scene occurs with their grandsons Joseph and Cesar in Part 2.
A heroic example in FLCL, where Canti powers up by devouring Naota, using him both as sort of a replacement for his missing parts and as living ammunition for his BFG form. He survives each time, though, and is eventually...er...passed out.
One of the block mechanics is Magic: The Gathering was called "Devour" and worked via your creatures coming into play and "eating" other creatures you had in play for various benefits.
Yu-Gi-Oh!: This is actually part of the overall mechanic. Monsters over Level 4 require you to Tribute one or more of your other monsters to summon them. And unless your entire deck is dedicated to the Weak, but Skilled, youwilldo this.
Not just Tribute Summoning. Fusion, Ritual, Synchro, and Xyz Monsters usually require at least one - and usually more - monsters to summon properly, and that's very often the case with most other monsters that are special summoned via their own effects. Long story short, this is such an important part of summoning monsters in the game that it's very hard to make a working strategy without doing it. (However, the anime almost never depicts this as a bad thing unless you hurt them on purpose while doing so, like Pandora did.)
Bloodscream, a sort of vampiric monster, partners with Cylla Markham to track down and kill Wolverine. When they find him, Bloodscream drains the blood from Cylla to replenish his strength, revealing that the only reason he'd brought her along was to feed on her.
During Av X, 5 of the X-Men each recieve a portion of the Pheonix Force. When one is knocked out, the others get their portion of the power. In the end, there are only two left, Emma Frost and Scott Summers. Scott is on the ropes, so he knocks Emma out, allowing him to become the complete Pheonix.
The Dark World arc of the Pony POV Series plays with this trope. When Discord is betrayed by Rancor and mortally wounded, Fluttercruel tries to invoke this trope, begging him to absorb her to heal himself. Discord refuses, since she's the one being he actually cares for, then inverts the trope by giving her his remaining power, allowing her to become a true Draconequus and the (apparent) Final Boss.
In A Different Lesson, Heian Chao attempts to do this with Wu Xiu. Interestingly, both are actually gaining from this, Xiu being healed from Chao's powers and Chao feeding off Xiu's chi. Fortunately, a thrown shield from Po (the Shield of Fire Monkey Pass, no less) severs the connection that is making this possible. as a result, Xiu is rendered comatose while Chao is hurled to the ceiling.
Connor MacLeod forces his younger kinsmen Duncan MacLeod to take his head and absorb his power, and thus have enough power to defeat the Big Bad. Sort of a Passing the Torch moment as well, as it's the hero from the original movie sacrificing himself for the good of the hero from the spinoff TV series.
Jacob Kell also kills his mooks to gain their energy. Had the film been edited together with any sort of care, this scene would have come after Duncan gaining Connor's power. Instead, it comes somewhat randomly near the end.
Hellraiser III: Hell On Earth. After Pinhead's shortlived right hand Monroe fails to offer Terri to him, Pinhead convinces her to instead take Monroe's place by his side and feed Monroe to him, which she obliges. The power is enough to give Pinhead humanoid form again.
Harry Potter: In the final book, Lord Voldemort does this with Severus, who while not second in command, was highly (wrongly) trusted, killing him to gain mastery over the elder wand. This was less effective than it could have been, as Snape was never its master
Although non-magical, Mr. Pin's killing of Mr. Tulip in The Truth is a similar case of a villain sacrificing his most trusted ally in order to shield himself.
Variation in the last book of The Wheel of Time, where Shaidar Haran is sacrificed to further weaken the Dark One's prison and allow it to manifest more fully in the physical world. What makes it unusual is that Shaidar Haran was actually a sort of weaker copy of the Dark One to begin with so the Dark One didn't so much steal his power as take back what had been given. As Moridin said, Shaidar was "A vessel of my Master, one that is no longer needed."
Power Rangers Lightspeed Rescue has Queen Bansheera get the last bit of power she needs to complete her transformation from a scarred and damaged near-cocoon like state to a fully mobile and powerful humanoid one by draining Vypra, citing her many failures as reason for this punishment.
In the adventure OA5 "Mad Monkey vs. the Dragon Claw", this is a special ability of Dragon Claw martial artists: the most senior one present can drain Hit Points from his fellow Dragon Claws, allowing him to fight longer. If the heroes fail, the Dragon Claw statue will do this to all Dragon Claw fighters in Kara Tur.
The Death Knell spell from Third Edition allows an evil cleric to do this to a dying comrade.
A number of powers of the Dark Pact, a warlock pact from Fourth Edition, have the warlock draining Hit Points from his allies to power his magic. Being as the Dark Pact was originally created by the Drow, who are natural backstabbers who hold absolutely no faith in friendship or fellowship... yeah.
During the Time of Troubles in the Forgotten Realms, Bhaal absorbed the souls of every assassin in the world in order to upgrade his avatar. To gain the strength to battle Bhaal's avatar, Torm likewise absorbed the souls of his own followers, but on a voluntary basis. This was done to justify the lack of an assassin class in 2nd Edition Dungeons & Dragons in-universe.
In the climax of the "Enemy Within" Warhammer campaign, the Big Bad drains lifeforce from the official he has been mind-controlling, allowing him to take his true form.
Seen in Resident Evil 4 as Salazar and his remaining Verdugo fuse with a Queen Plaga before his fight with Leon.
In Resident Evil 5, Wesker injects Excella with the Uroboros parasite, which devours her from within.
Inverted in Final Fantasy X, when one of Seymour's incarnations features "morticorpus", a living shield that regenerates by draining his life.
He does play this straight right before that Boss Battle, though, by absorbing the body of the recently deceased Maester Kinoc (who was killed by Seymour offscreen shortly before) along with four of his own mooks in order to jumpstart his transformation into the One-Winged Angel form that includes the morticorpus.
In Legend of Mana, after the Hero beats Lord of Jewels 999, its dragon Sandra offers it her core gemstone so it could be reborn as Lord of Jewels 1000.
In Dragon Age: Origins, a Blood Mage can drain health from other party members with "Blood Sacrifice", which is a good way to keep the Blood Mage alive since keeping "Blood Magic" active renders other forms of healing all but useless. If you're careless it's entirely possible to bring your ally to the brink of death.
Inverted at the end of Silent Hill 3: Claudia devours the embryonic God to enable its birth, which of course destroys her.
In The Legend of Zelda: Spirit Tracks, Big Bad Malladus was forced outside of Princess Zelda's body. With no convenient way to repossess her, Malladus opts to simply possess his not-as-compatible Dragon, Chancellor Cole, creating an unstable beast that serves as the Final Boss.
In Soul Nomad & the World Eaters, Thuris drains Kanan, the leader of his cult for a power boost before his boss battle after the rest of his cult wasn't enough.
In Soul Calibur 4, Tira wants Nightmare/Soul Edge to do this to her because she has a bad case of Mad Love for it. In her ending, the opposite happens. Nightmare's body can no longer withstand its own power and dies while Tira begs him to stay with her. Tira gets her wish when Soul Edge's power merges with her.
In the Dual Boss battle against Ornstein and Smough in Dark Souls, defeating one of them will cause the other to regenerate his health and absorb his fallen comrade's power. Smough gets Ornstein's lightning, while Ornstein becomes gigantic.
Star Wars: The Old Republic: The Sith Inquisitor has the "Sacrifice" ability, which sacrifices your companion in order to restore health to yourself. Depending on how you play your character, your companion might indeed be The Dragon to your Big Bad. The Inquisitor also consumes force ghosts during the story to gain the power to confront his/her nemesis.
All classes get this ability, if they get to level cap and full dark-side.
In Prototype 2, Alex Mercer prepares for the final battle by consuming his Evolved lieutenants.
In the final battle against Miraak in The Elder Scrolls V: Skyrim DLC Dragonborn, Miraak fights you while three dragons fly nearby. Each time he is brought near death he will absorb one of his dragons' souls to restore his health.
In Haunting Ground, a New Game Plus cutscene shows Lorenzo, a creepy old paraplegic and the real Big Bad of the game, crawling over to the defeated body of Ricardo. He sucks out Ricardo's Azoth through his mouth. Though its effects are not immediate, this is why Lorenzo starts off as a crazy old man and ends the game as a nearly indestructible young sorcerer. Azoth is the source of life in alchemy.
During the Final Boss fight in Kid Radd, Radd is brought down to 1 health and is about to be finished off with an unavoidable attack when Bogey forces Radd to kill him so that he can get a health power-up.
Implied to have happened in There Will Be Brawl, when Kirby is revealed to still be alive, there are no signs of either of his Dragons, and he's wearing the cap of one of them. Now, remember that Kirby's powers involve copying the powers of other characters by literally eating said characters, and he usually dons a hat reflecting said powers...
As noted in the page quote, Makuta absorbs Nidhiki and Krekka in BIONICLE: Legends of Metru Nui, as well as his pet Feathered Fiend Nivawk. This was a mixed blessing however; he later claimed that he had trouble suppressing their minds within his own, distracting him enough for the heroes to win.
In Wakfu, Goultard dominates the fight against Rushu. Realizing he's in a bad position, Rushu calls upon his Shushu hordes to help him, and they merge into a huge mass of darkness. Rushu eats it to get a power boost that allows him to gain the upper hand against Goultard.
A literal example in Dinofroz. Before the final battle, dragon king Neceron zaps his three generals to gain their powers, leaving them dying. Oddly enough, despite him explicitely saying that they will die without their powers, the three dragons appear later alive and well, although two of them die for different reasons.