In Bleach, Hollows become more powerful by eating each other (and human souls) at lower stages of strength, this normally leads to them becoming beastial creatures known as Gillians, composed of many thousands of souls and forming the first stage of a Hollow's existence as a Menos. Occasionally one of these hollows contain a soul strong willed enough to remain sapient and somewhat intelligent, which takes control and continues to feed on other hollows to become more powerful. If such a Gillian consumes enough souls, it turns into a more powerful and now-truly intelligent Adjuchas.
Naraku from InuYasha was created when the deranged and perverse thief (and Kikyou's Stalker with a Crush) Onigumo merged with thousands of demons. And at one point in order to obtain a stronger body, he perfoms a Kodoku spell: thousands of fierce demons fight inside a cave in a mountain, and the winner absorb and merge with the losers. Eventually Naraku ends up absorbing the surviving demon in himself.
Dragon Ball Z: By putting his hand on Nail and later Kami, Piccolo absorbed them, gaining not only extra power but also their memories. This process was permanent; Piccolo's personality also seemed to underway slight changes after each absortion. A similar case was when Goku and Vegeta used the Potala earrings, becoming one; however, in this case they split when they entered Buu's body, although it's the only way to split, and that fusion is otherwise permanent. The personality of the fused being seemed to be a mix of both Goku and Vegeta's personalities.
In Shinzo, all Enterrans, the new inhabitants of Earth, are reduced to an Encard when defeated. The victor can then decide to keep the card, destroy it (effectively destroying that Enterran's soul) or eat or absord it. The soul and power of the eaten Enterran are then merged with the eater's, who usually remains in control but does take over some of the physical characteristics of the other Enterran.
In Saint Seiya: The Lost Canvas, the Oneroi, four dream gods, have a One-Winged Angel form that fuses four of their souls into a single body. While each souls is still separate, they work in tandem to keep the body from being destroyed by harm and grant it Nigh-Invulnerability. The catch? Attacking all four souls simultaneously with precision strikes can kill them.
An arc of DC's Hitman comic featured the Mauser, a demonic minion created by merging the souls of several Nazi S.S. officers.
Redlance and Nightfall in ElfQuest have their souls merged as an accidental side-effect of Nightfall's love bringing Redlance out of a Heroic BSOD. They get to keep their individual personalities, though.
In "What If The Avengers Had Lost the Evolutionary War?" from the second volume of What If?, many of the superhumans cut their ties to humanity and leave Earth find their own destiny. Eventually, they merge with the Anthropomorphic Personifications of Death and Eternity, and the resulting entity leaves to create a big bang for a new universe, afterwhich it splits into just Death and Eternity. Meanwhile, humanity evolves to the point that the humans merge their minds and become one with the planet itself, transforming themselves into a Genius Loci.
Jim Henson's The Dark Crystal had the wise Mystics and the wicked Skeksis, who were created when the titular crystal shattered. They were two halves of the souls of the creatures which shattered the crystal, and a death of a Mystic also kills their Skeksis counterpart, and vice versa. In the climax of the film, the crystal is mended by the hero Jen, and the two races are made one once more.
Happens towards the end of Isaac Asimov's "The Last Question", when the remaining humans (by now bodiless entities) merge one by one with humanity's hypercomputer, the Cosmic AC.
The Greek philosopher Plotinus (c. 204-270 AD) postulated "Emanation ex deo (Out of God)". Basically, in the hirarchy of being, there is The One (who is all good, transcended, and unchanging). The nature of the One is simply that it filters down itself, but the One never loses anything or changes. Next comes the Novus, or Divine Mind, and then bellow that is the Oversoul. From the Oversoul comes individual Human Souls. So what does this have to do with this trope? It's possible for a human soul to reunite with the One again, forever (at least in Neoplatonic tradition).
Terry Pratchett's Discworld: Thief of Time introduces Lobsang Ludd and his not-quite-twin brother Jeremy Clockson. The awful truth, when it comes out, is that they are not twins at all but the same soul born twice due to a temporal anomaly (their mother is the anthropomorphic personification of Time, and her labour got a bit strange, despite the best midwife in the Discworld being present). The two half-souls are allowed to fuse together towards the end of the book, with Lobsang dominating, so as to take over the job of running Time from their mother.
In George Zebrowski's Macrolife, The Agregate of Minds is a distributed artificial intelligence (spread amungst a fleet of a million or so artificial worlds.) whose countless processors and memory storage systems apparently include a library of simulated personalities. It is apparently capable of reversing the process, allowing a component intelligence to function independently in order to hold conversations and then reabsorbing it when done.
In The Gods Themselves, this is how the Soft Ones in the parallel universe have sex. It's also how the Hard Ones are formed, when the last mating becomes permanent.
Echo and Alpha from Dollhouse both have 40+ personalities inhabiting a single body. It is clearly shown in Echo's case that her "main" personality is an amalgamation of all the personalities she has been imprinted with, though she can seamlessly slip into one specific personality when needed. With Alpha, it's not as clearly defined and there are even a couple scenes where two or more of his personalities are openly arguing with each other. He does seem to have one "main" personality that is in overall control, though it is never clearly shown if this "main" personality is an amalgamation of all his personalities like Echo's is.
Castiel, an angel from Supernatural, became something far more powerful by absorbing the souls of Purgatory. Unfortunately one set of them was too powerful for him and took over.
The Faithless song "We Come 1" deals with the merging of the speaker and the one addressed.
The God Emperor of Mankind was said to be the reincarnation of thousands of prehistoric shamans' souls, merged into a singular being.
Slaanesh, an Eldritch Abomination and one of the setting's fourGods Of Evil, was born when countless Eldar souls that died in extremely decadent orgies and sadistic slaughters coalesced together into a single entity.
Towards the end of Dust: An Elysian Tail it's revealed that Dust himself has the souls of Ginger's brother and the Big Bad's henchman/best friend who slew each other. It's explicitly stated that the resulting being has its own mind and body, with either soul just providing moral guidance and power respectively.
Exdeath, the Big Bad of Final Fantasy V, came about after several beings and creatures were sealed within a tree in the Forest of Moore.
The first game's high templars can perform a one-way fusion into an archon, a ghostly psionic entity that is very damaging and quite hard to kill[[labelnote:*]]barring use of the science bessel's EMP, which strips their shields to leave them with just a small amount of HP[[/labelnote]]. The Brood War expansion allows two dark templar to fuse into a dark archon, a caster unit.
StarCraft II dispenses with the dark archon, allowing archons (initially called twilight archons, but this was dropped in early patches) to be created from any combination of two high templar or dark templar.
Shang Tsung's power is said to be derived from his having absorbed the souls of his vanquished opponents. He is dangerous because he is not one opponent, he is thousands.
Ermac, who is made up of the thousands of souls whose shells died resisting Shao kahn's rule
EverQuest II introduces the Duality. The combined essences of the great wizard Al'Kabor and a powerful necromancer named Dartain sharing one body. His physical body morphs between the two personalities at frequent intervals.
In Neverwinter Nights 2: Mask of the Betrayer the Thayan Academy of Shapers and Binders has a wizard who can fuse souls, and bound his own soul into a clay golem to do it more safely. A sidequest on the same level requires you to trade a particular soul to two pit fiends. Unfortunately, they can't agree on what the soul should have; in fact each gives mutually exclusive requirements. The solution ends up being to have the aforementioned wizard fuse two completely opposite souls together.
At the end of the undead campaign in Warcraft III: The Frozen ThroneArthas merges his soul with the Lich King.
The Generation IVPokémon Spiritomb is a collection of 108 souls that have amalgamated together out of negative emotions.
There's conflicting evidence for this and Mind Hive in the case of Mass Effect's Reapers. They're created from the amalgamation of millions of sapient minds but it's unclear whether the souls are fused or exist separately: Legion states the latter, but Reapers act like single entities and refer to themselves with singular pronouns (contrast Legion, who always uses "we").
Acibek was a golem created by an ancient Elven tyrant who powered him with several souls combined together, many of them collected unwillingly, and not happy about it. The Sylvan Oracle and her own creation "Dirk the Mighty" were similar golems.
And there's also the Storm of Souls, a raging maelstrom of souls formed from deceased members of the chaos-worshipping cult The Chosen and intended to be their weapon to destroy the world with. Acibek sacrificed himself to trap it in a cage forged from his own souls and centuries later the Sylvan Oracle sacrificed herself to destroy it.
Homestuck: Vriska's and Tavros' dead bodies are thrown into Jane's game sprite, yanking their souls out of the afterlife and fusing them together as "Tavrisprite". Their personalities are so irreconcilable that the sprite explodes (sending the two souls back to the afterlife) mere moments later.
This happens in Spinnerette, unfortunately the souls are evil souls from Hell merging together into a Kaiju-sized demon.
In Orion's Arm, "The Amalgamation" is an artificially intelligent disease, spread by Nanomachines. Occasionally, crews of ships sent to aid efforts to halt Amalgamation become infected and turn against their allies.
There was an episode of The Real Ghostbusters featuring an advanced anti-ghost robot designed to destroy (actually disperse) ghosts instead of capturing them. As soon as the dispersed essence concentration went above a certain threshold, well◊...
Tigerhawk in Beast Wars was originally a Vok Emissary made from Tigatron and Airazor's bodies. However, their Sparks followed their bodies, and after Tigerhawk was defeated, Airazor and Tigatron's sparks merged and took control of the shell.
Five hats means that five tropers think it is ready to publish.
You are saying that you think this draft is ready to be published. That means the description is not ambiguous,
it doesn't duplicate an existing trope, there are at least three examples, and the title makes sense.
Is that what you meant to do?
You are saying this draft has a ready-to-publish hat it does not deserve and you are taking it back.