This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / The Elder Scrolls IV: Oblivion
The whole game is one, albeit in a more subtle and melancholic manner. First, Uriel Septim, who was emperor during all the previous games, and whom you have helped in your various incarnations as a PC, is slain. The Septim Empire, which united the whole of Tamriel is facing a terminal decline. And to top it all off, remember all of your favourite places, people, guilds/societies and else? They will most likely be destroyed or killed in the ensuing equivalent of WW2 ("The Great War") against the Third Aldmeri Dominion.
The quest A Brotherhood Betrayed. Three adventurers found an ancient amulet, but one wanted it for himself. So after many years, that traitorous adventurer murdered the other two in the guise of a vampire hunter, setting the other two up as vampires. The wife of one of the victims, shocked by what had happened, asks you to help. After the truth has been revealed and the seemingly mundane amulet retrieved, she reveals that the other two knew what the traitorous adventurer had been planning, and cursed the amulet so it becomes mundane. The secret word to lift the curse? Brotherhood.
You can find a dead troll under a bridge. Funny? Yes, until you read the note on its body, confirming that, yes, all those trolls you killed for their fat are sentient.
"Mee wurst troll evurr
nobuddy pay brijj tole
me nott sceary enuf
mee gett drunc an kil sellf
The beginning. When you first meet Martin in the chapel of ruined Kvatch, before he even knows who you are or he is, he just sounds so weary and disillusioned. Possibly even Harsher in Hindsight, when you know how it all turns out.
"Yes. I'm a priest. Do you need a priest? I don't think I'll be much help to you. I'm having trouble understanding the gods right now. If all this is part of a divine plan, I'm not sure I want to have anything to do with it."
The ultimate fate of Martin Septim, who sacrifices himself to become an Avatar for Akatosh to permanently defeat Mehrunes Dagon and seal off Oblivion forever. This counts as a tearjerker, since aside from the potential of him being a great Emperor, Martin was himself a good man, who was the real hero of the story, not the player character. It's not only heartbreaking, but frustrating that all your work protecting him from the Daedra is in vain.
Perhaps the worst thing about the ending is how everyone calls you the Hero of Cyrodiil after the deed is done. Why is that so bad? Because you didn't do a damned thing. In the end, all you did was help Martin sacrifice himself.
What's worst is the Harsher in Hindsight aspect, when you see the state of the world in The Elder Scrolls V: Skyrim. It almost makes you wonder if all the struggling your Oblivion character had been through to save Tamriel from Dagon had all been for naught...
Agronak gro-Malog's discovery that his father was a vampire leads him into a severe identity crisis and ultimately suicidal depression that causes him to forfeit the Grand Championship match, allowing you to kill him without him fighting back. Up till then, the Gray Prince has been a jovial fellow, if a little worried about his past. You will feel like a bastard for causing his disillusionment with himself, and for taking advantage of his depression to become Grand Champion without a fight. It lends a very bittersweet feeling to the end of the Arena questline, especially when the Blue Team Gladiator, who had been nothing but amicable until that point, angrily calls you out on murdering Agronak in cold blood.
Even worse, killing Agronak this way counts in-game as a murder. Which means that the next time you sleep, you WILL be contacted by Lucien of the Dark Brotherhood. This is often the Start of Darkness for players that were previously seen in-game as only a hero to all.
Even if you decide to not reveal Agronak gro-Malog's true identity and instead kill him in a fair fight, the Blue Team Gladiator still calls you out for murdering his friend and mentor. It can really make you feel guilty about an otherwise awesome victory.
"Congratulations, Grand Champion. I hope it was worth it."
It's easy to find the Dark Brotherhood (assassin) missions a little heartbreaking. Why? Well, when you arrive at the Sanctuary after your initiation killing, you are quickly introduced to a rather (disturbingly?) cheerful band of murderers, whom the mission-givers encourage you to talk to for advice on your "contracts". So after a while, you've gotten to know all of them, their quirks, even the mean old shopkeeper, and the black humor surrounding the missions themselves is hilarious ("If [soon-to-be-dead guy] doesn't quit that awful drug, it'll be the death of him!"), and then you get to the halfway point. The guy who recruited you thinks there's a traitor amongst them, and in order to get the traitor, he orders you to kill everyone in the sanctuary, who have come to love you like family. All of them. Even darling Antoinette-Marie. And you have to, if you want to complete the mission line. And you'd have to be completely soulless not to feel bad about it. The jerkass shopkeeper M'raaj-Dar chooses now of all times to start being nice to you where he decides that the two of you got off to a bad start and wants to try and be friends! Just to add insult to injury, it seems like the rumor mill chooses that moment to pick up all the rumors about all the new dead guys you had a hand in making. It's enough to make you pull a Heel–Face Turn...
This is made worse by the fact that you later find out there was no traitor — at least not in the Cheydinhal Sanctuary. You killed them all for nothing.
When you're too late to save Lucien, and you never get to clear his name, either.
Some other Dark Brotherhood targets are fairly likeable too. Matilde, "Next of Kin" or "Honour Thy Mother" are good candidates for those that go too far. They're among the worst things you're ever asked to do in any video game in a serious context.
Another Dark Brotherhood quest, "The Lonely Wanderer". To find Faelian, you have to ask around for info, and end up at the King and Queen Lodge in the Talos Plaza District of the Imperial City. You talk to the innkeeper there about him, and she mentions how he has a wealthy girlfriend that's apparently devoted to him. You talk to the girlfriend (possibly needing to chat her up into a friendlier level), and SHE tells you all about how even though Faelian's a penniless Skooma addict, she still loves him and hopes he manages to someday clean himself up before he gets himself killed.
Reading Viranus Donton's journal. He writes about how he thinks his mother doesn't think he's as good as his recently-killed brother. You find the journal when you find him dead in a mine. The final entry? "I hear trolls I'm sorry Mother".
The penultimate quest of the Fighter's Guild requires you to infiltrate the Blackwood Company and learn their secrets. As part of the quest, you're required to drink a bottle of Hist Sap, a hallucinogenic drug, and help the Blackwood Company clear out a town of a goblin infestation. When you return to the town after the drug wears off, you find that in your hallucination you mistook the innocent inhabitants and their sheep for goblins and slaughtered everyone in the town, including Biene Amelion, who you helped out in a previous quest. Her father, who you freed from jail by helping cover the cost of his bail, wonders what kind of monster would do such a thing...
The end of the Thieves' Guild quest line when you meet the Gray Fox in Anvil and he reveals himself to be Countess Umbranox's long lost husband, Corvus.
The entirety of the Shivering Isles tends to be depressing, if not heartbreaking. To start, we have a realm that invites its users in with a laughing Sean Connery-like voice. Immediately after, it will either drive the "guest" insane, and spit him back out to be killed, or keep him/her there forever, thus cutting him off from his life. Next, for those that survive, there's the Gatekeeper's death (which isn't so bad as far as these things go) but is compounded by the fact that you can use its mother's tears to injure it further. The crying alone deserves a special mention, because that hints that the mother knows her child is going to die, and that she has seen it before, and she knows she's bound to making these things, just to see them die. Further in, we have an entire city divided into insanity, and the Dementia side is truly heart-wrenching. The people within are bound to a lifetime of sorrow and paranoia and the only way out (suicide) is punished by magical suicide cliffs that bind the souls of those who leap from them, only to be sentenced to stay there for all of eternity. To wrap this happy little party up, Sheogorath himself is depressing, as he is bound to continuously watch as his world is built up, and then see himself tear it down piece by piece, only to start again.
Near the end, as Sheogorath turns into Jyggalag. Hearing the wacky, crazy, eternally-cheerful Sheogorath speaking in an orderly, logical manner about his realm dying and sounding so depressed... And then he sadly tells you to get out while you can, or he'll kill you. Whether you see him as a painter forced to burn his art or he actually cares about his people either way it's pretty horrible.
And to add another layer of horror to it, anyone with experience with Dementia or Alzheimers can be swiftly reminded. In Sheogorath's case you loved him with his eccentricities and insanity but you're forced to watch this person become sane and you are absolutely powerless to save him.
Sheogorath: Time, Time is an artificial construct, an arbitrary system based on the idea that events occur in a linear direction at all times. Always forward, never back. Is the concept of time correct? Is time relevant? It matters not one way or another. I fear our time has run out.
Later on, the consequences of this event forces you to Shoot the Dog. However, this means you technically [[spoiler: kill a GOD, so it could also double as the player character's Crowning Moment of Awesome.
The 'Final Resting' quest, where you are asked to put someone out of their misery. The suicide note is just the icing on the cake.
Even some minor details can provoke this. There's a cave in the Shivering Isles populated by elytra and gnarls. Toward the back, you will find a dead woman. She is holding a diary, which details her coming to the cave with her lover and him wishing to make the cave their home due to his finding other people stifling. She came to find the cave something of a prison since she could not leave without breaking his heart (and it having been populated by the aforementioned monsters). She planned to escape, but confronted him about it first, and he allowed her to leave. However, she broke her leg on the way out, and was unable to either leave or return, and he did not respond to her cries for help. Thus ends her story. Deeper in the cave, you come across the man himself - he is still alive but attacks you on sight. If you kill him, and read his journal, he details the same events from his perspective, including how heartbroken he was that she wanted to leave. His diary finished with him hearing her voice, but putting it down to the echoes of memory, not realising she is genuinely in mortal peril. There is no quest relating to these two; you will only encounter the story at all if you find and choose to explore this cave.