This is based on opinion. Please don't list it on a work's trope example list.
Tearjerker / Guild Wars 2
Each of your Order mentor's death:
Magister Sieran (Durmand Priory): A very happy-go-lucky Sylvari who was in love with the world, and had a tendency to say 'cherry!' when she was happy about something. She treats her death as another adventure, and says she is excited to see what awaits her in the Mists, but you can tell she's terrified.
Lightbringer Tybalt Leftpaw (Order of Whispers): A very easygoing Charr engineer, who (despite being a member of the Order for some time) does his first field mission with the Player Character, and rejoices when he is assigned to permanent field duty. A deep friendship slowly grows between him and the player character – with this odd Charr who has a fondness for apples, has insecurities linked to his missing right paw (which was blown up a few years ago), and who eventually reveals to the PC that his partnership with them is what gives him the courage to stay in the field. He sacrifices himself in order to help you escape from Claw Island. During a later mission defending the Order of Whispers from a Risen assault, it is revealed that the security system was designed by Tybalt, and that the password to override the system is apples.
Warmaster Forgal Kernsson (Vigil): In contrast to the other two, Forgal acts more as a stern father-figure to your character than a good friend. He softens up gradually, showing more and more how proud he is of you; until you learn at Claw Island that he had previously lost his family, meaning that he as a Norn had no one to tell and pass on his legend to until you came along. He thus decides that keeping back the Risen while you escape will be a worthy death, and entrusts you to tell it while he walks back into battle against a dragon champion and a huge number of Risen.
This comes back to haunt your player character in the penultimate story mission where you fight ANOTHER GODDAMN EYE OF ZHAITAN. In addition to not wanting to fight another goddamn Eye of Zhaitan for the nth time, it taunts you about your dead mentor and says that they are "waiting for you now, beneath the dragon's wings..."
The fact that a number of the allies (Lionguard Apatia from Claw Island, Kekt from the undead ambush in Ural's Delve and so on) you met, usually from earlier missions, started dying in the personal story missions for levels around 70-80 is just really, really sad...
It gets sadder when your other allies have put all their trust in that person or when you find out they've been turned into Risen or when you find out that that person looked up to your Player Character.
The events that transpire if, during the mission 'A Light in the Darkness', you told the Pale Tree that your greatest fear is "letting an innocent die", in particular 'A Sad Duty'. At the end of the mission chain leading up to the Battle of Fort Trinity, your partner, the Asuran demolitionist Tonn, sacrifices his life to destroy the Orrian Bone Ship, swimming back out to fix the explosive charges and telling you to set them off while he's still in range. After the Battle at Fort Trinity, you accept the duty of telling his beloved wife Ceera (a Pact field medic who Tonn constantly discussed with you fondly) of her husband's death. The news destroys her, almost driving her to suicideto join him and she swears never to forgive you for causing his death. While you manage to save her, when you leave her she's helplessly sobbing her eyes out alone. For those who question whether the Asura are capable of either true heroism or truly caring about anyone other than themselves, this mission will set you heartrendingly straight.
General Tyria, dungeons and expanded universe:
Snaff's death, and the disbandment of Destiny's Edge. Along with the fact that the dragon Glint is also dead. The events closely following after the deaths were quite heartwrenching for this troper.
Eir being told that her whole plan failed and that she killed Snaff.
Eir: You can't die! Zojja: He's already dead. Eir: You can't die. (takes Snaff's body from his golem and cradles it) Zojja: Put him down! You have no right! Your plan failed. You killed him! Eir: I killed him?
Zojja having to cremate her own master's body, and her exchange of words with Caithe after gathering his ashes:
Caithe: Rytlock is leaving. Zojja: Just like Logan. Caithe: We have to stop him, or go with him. Zojja: I don't have to do anything. Caithe: Don't be irrational. Zojja: Who are you to tell me anything? You're not my master, my master is dead. Caithe: This could be the death of the whole world. Zojja: My world is dead.
Rytlock realizing that all their efforts were pointless, and that he was a fool for trusting Logan.
Caithe's efforts on trying to keep the group together, which miserably failed. Until now, at the present time of the game, she still attempts to do so. And all those attempts failed as well. In a different timeline when the Pale Tree presents a vision of Orr to the Player Character, you find that she's grown so tired of rounding them up only to fail again that she decides to join Faolain in the Nightmare Court.
One particular event in Straits of Devastation requires you to clear out a field hospital of Risen. After the event is completed, we get this exchange:
Commander Blaike: Status report. How many of our sick and injured did we lose? Medic Kellye: All of them... sir. Commander Blaike: I... I see. Repair the damage and prepare to receive more casualties.
What really sells it is the way Blaike's second line is delivered, with the perfect blend of horror and realization, yet still remaining partly stoic in the face of crisis.
The dungeon instance Honor of the Waves. You follow a Kodan Claw (protector-leader) through the wreck of his floating sanctuary overran by icebrood. You first arrive too late to save the Voice of Koda (spiritual leader, with whom the Claw forms an inseparable pair), who is corrupted by the Dragon and has to be put down by yourself and the Claw, and then, in one of the explorable paths, you have to slay two corrupted icebrood Kodan who form an honor guard to a Svanir fanatic. The Claw identifies them as his sons.
Interesting to note is, his name is never given. We know Kodan choose their names to match their nature (and that name may change during the course of life), and while we have a plenty of named Kodan, this one is only known as "Honor's Claw".
The sight of fallen refugees in the first Living Story update. Flame and Frost added a few NPCs around refugee camps in their cities. One of them, the Refugee Primus, mentions how the cubs he was watching over were killed. Also, the general idea of the refugees being completely beaten down by the disaster and forced into rat-infested camps in their capitals.
The November '13 addition, the Living Story "Fractured", has one near the end – when you realize the hub instance you go through every time when you're in the Fractals of the Mists is nothing else than Dessa's own personal fractal.Just before that, Dessa muses on the fate of her krewmates from Thaumanova, who have to relive the same moment in their lives (in their case, death) every time someone activates their fractal.
Lion's Arch in the aftermath of Scarlet's attack. The place is in ruins, the music is very sad in tone; and lots of NPCs are either grieving, confused, have left notes for lost loved ones or are digging graves for the fallen.
There was a noticeboard where survivors left notes to try and reunite with missing loved ones. After reading a few, the last one is written in the handwriting of a young child, and it says "I can't find my mommy or daddy."
Heart of Thorns:
The way the Sylvari are being treated. This varies from suspicion and coldness to Pact vigilantes kidnapping Sylvari troops to murder them. It's hard not to feel sorry for them.
Pale Reaver Scout: Please, don't hurt me.
Some characters actively refuse to treat Sylvari this way, however, and the contrast works wonders for botheffects.
It's not just the treatment of Sylvari, it's their situation in general. Permanently having a voice in their head that drives them insane enough to turn on their comrades. Those who haven't "turned" yet are terrified of the prospect, and even a hardened soldiers like Laranthir or cynics like Canach are visibly shaken whenever they are reminded how close they really are of losing themselves.
The death of Eir Stegalkin, and Braham's reaction.
To add to this, Garm is still missing.
Remember Ceera, the Pact medic mentioned above? You meet her again, at the entrance of Auric Basin. Turns out Tonn died during the war against Zhaitan regardless of your choices. She mourns him, and is still (at least a year later) clearly devastated by the loss, but is determined to do him proud by working hard for the Pact. She talks to the Player Character, giving some tidbits of exposition about the area. Unless youas a commander were "responsible" for Tonn's death (i.e. played that particular branch of original personal story) – in that case, she won't have a word with you. The hatred and vitriol her lines are delivered with in that case are chilling.
Ceera': Commander of death. I have no words for you.
In the area nearby, you can find and recover dead bodies of Pact soldiers, two for each race, always male and female. Upon finding them, your character, no matter what race they themselves are, deliver a simple and short eulogy appropriate for the race of the fallen soldier (charr making their Legion proud, Norn having their legend go on, Asura being part of the Eternal Alchemy, etc.). Even the male charr player's voice actor (gruff, growly, and due to race, hardly ever emotional about anything) sounds solemn and sad at this point.
The revelation that Rata Novus is nothing but a dead city. Taimi sounds so hopeful, so excited. But unless you played through the new maps only with the personal story, you already know she'll only find a hollow ruin overran with chak.
The scene where Taimi gives up Scruffy. Making it even worse is how Scruffy hesitates, apparently needing reassurance.
Finding Logan and Zojja is both reassuring and heartbreaking. They have clearly suffered greatly.
In the final battle Mordremoth summons versions of the companions you left behind in other to fight you. Their dialogue is one of despair while keeping their personalities. But the worst is when he summons your Order's mentor (Tybalt, Sieran or Forgal). The commander is absolutely pissed at this action. A happier version happens at the end when you purify the phantom and they join you in the fight.
Tybalt Leftpaw: It's good to see you, my friend. Player Character: I've missed you, Tybalt. Grab an apple and let's finish this! ** What Mordremoth is doing in that stage is activelly leeching your character's memory for things to make you hurt and wreck your morale. He only finds your mentor because the Commander still vividly remembers and is hurt by their loss.
Finding Trahearne, who is too far gone to save. He just sounds so exhausted. His friend, the Commander, is tasked with killing him with Caladbolg. The beginning of 'Fear Not This Night' plays softly as Trahearne asks for the sword.
The first ever raid wing, Spirit Vale has a personified nightmare and tearjerker all merged into one, and its name is Gorseval the Multifarious. Merged tormented souls of humans that feed off other souls, constantly adding more to their collection. During the battle, it calls on you in multiple voices, all of them hateful and insane.
Later, it turns out the bandit leader Sabetha has been using Gorseval to dispose of the bodies of the Pact soldiers and civilians she had abducted. At some point, she fed it to such strength that the bandits themselves were unable to go anywhere near it and had to find a different way to dump the bodies, burning them in pits instead.
The path to Gorseval leads through an old human (desecrated) mass grave and cemetery. The souls there haunt you. In every sense of that word.
In contrast to that, in Salvation Pass, you also find souls (marked ??? in the chat log) of the fallen humans. These in turn are terrified of you, or they call to you and plead you to come back just as they disappear.
Later on, you discover that Matthias Gabrel, the final boss of the wing and inquisitor of White Mantle, sacrificed them all, to feed a ritual with their lifeforce and gain more power for himself. Said power is in turn becoming so uncontrollable even his own men are terrified of how unhinged their boss has become.