I'm not going to spoiler-warning this, it's WoW content from Vanilla, and very much no longer in a position where you can properly experience the story:
Nefarian - prince of the Black Dragonflight - is one of the first Big Bads in WoW. You first encounter him when exploring a late-game dungeon, Blackrock Spire, which is split into two parts - the Lower Spire, designed for five, and the Upper Spire, originally designed for fifteen (and later reduced to ten) people; not a full-size raid, but a bit tougher than a conventional five-man. The Upper Spire, however, is locked behind a quest chain which starts in the Lower Spire.
You meet Vaelan - at first disguised as a Blackrock Orc - while in the Lower Spire, and he's clearly on your side and working against Nefarian. He helps you proceed through the quest chain to open the door to the Upper Spire. Towards the end of that chain he reveals his true form: Vaelestrasz, a member of the (good) Red Dragonflight:
As you proceed into Upper Blackrock Spire, halfway through the dungeon you drop down into an arena for a protracted fight against Rend Blackhand, with Victor Nefarius - Nefarian in human form - watching from a balcony. During that fight you can summon Vael to help you, after which he'll pursue Nefarian through a raid portal into Blackwing Lair.
Later, when you're on Blackwing Lair, you'll encounter Vaelestrasz again, but he has clearly been fighting against Nefarian and has lost; collapsed, too exhausted to continue, with Nefarian draining his energy. On you turning up and spoiling the party, though, Nefarian runs off, leaving Vaelestrasz corrupted. In Vaelestrasz's last throes of sanity, he
gives you a buff to help you kill him, before being completely consumed by uncontrollable rage and attacking your party.
Not only do you have to kill him,
he helps you do so.
Oh, and for good measure, the aspects of him that are in control result in him constantly apologising as the fight goes on.
Forgive me, (player)! Your death only adds to my failure!
