In-game both Horde and Alliance players fight raid bosses, but sometimes the game’s lore has one or the other as the definitive killers. Ex. In vanilla both sides can fight Onyxia in-game, but the Alliance (mainly Varian) are the ones that canonically kill her.
I was wondering how many raids/raid bosses we know the canon endings of.
Chronicles 3 has your answers, up to cata. In lieu of spending 15 minutes writing it up, here’s it all from wowhead:
https://www.wowhead.com/news=283053/warcraft-chronicle-volume-3-new-canon-info-on-champions-of-each-dungeon-raid
This is the exact kinda thing I was looking for thank you!
I tend to dislike the canonizing raid situation myself, as while sometimes it makes sense (Alliance taking out Onyxia) othertimes i feel it makes no sense (Alliance being the only guys at the sunwell)
I really wish that some of these dungeons and raid that say both sides participated would allow both sides to do so in game. We really should have been able to in legion.
If it’s an important raid, the Alliance cleared it.
We both know the horde isn’t allowed to have nice things. God forbid they outshine the alliance just ONCE in a expansion.
Wouldn’t that make sense because of Velen?
Why does Velen make any sense being there? It’s the Blood Elven homeland with the most important Blood Elven artifact in all of Blood Elven history, which the Blood Elven prince sold to the Burning Legion in the betrayal of the Blood Elves to damn them all.
Velen being there makes sense, what doesn’t make sense is why the blood elves not being involved. I think most raids, especially major ones, should be a joint effort.
It kind of cheapens Velen’s revelation regarding the Naaru and the redemption of the Sunwell, when the Sin’dorei who it belongs to would supposedly not have any involvement in its purification, no?
It feels like they flipped a coin to see which faction (and ONLY that faction!!!) cleared which place.
Both the Alliance and Horde fight on Quel’Danas. The Horde defeated Magisters’ Terrace, in which they faced Kael’Thas a second time. He’d survived the assault on Tempest Keep, capturing M’uru and Anveena to repower the Sunwell to serve as a gateway for Kil’jaeden. The Alliance infiltrated Sunwell Plateau, driving Kil’jaeden back through his portal after the heroic sacrifice of Anveena. Velen then used the heart of M’uru to cleanse the Sunwell, transforming it into a font of holy magic which Lady Liadrin and the Blood Knights embraced.
Yes. Only the Alliance were present for the Sunwell raid. Magister’s Terrace isn’t even near the Sunwell. They gave the Horde an unimportant dungeon and the Alliance the important Kil’jaeden defeat.
And yet there are still people claiming blizz favors the horde more. Seems every major victory and awesome scene is given to the alliance.
Technically chronicles has become no longer canon so you can say it was the horde.
I just see it as an alliance force because of Velen and they already rationalized the horde fighting Kael’thas.
I don’t think it fixes the fundemental issues of assigning raids a canon faction, it being most raids are important for both factions in the story. Some like Uldir and the Shrine of Storms make more sense for one or another, while others like An’Qiraj and the Sunwell don’t.
Yeah, the majority of locations in chronicles don’t make sense that way.
It has always bugged me that Lor’themar wasn’t at the Sunwell if Velen was going to appear like that, I might hold that opinion just because I consider the handling on the horde has been shotty.
The Blood Elves had Liadrin there, likely do to their mutual connection to the light and the fact they used the Naaru she drained to relight it
The blood knights are now part of the horde, perhaps i’ve been holding a grudge against the devs. 
edit: Though most alliance groups are considered neutral like Tyrande with the centurions which could explain why I overlook Liadrin’s involvements.
Neither faction was at the Sunwell, at least not in the name of their faction. Everyone was working under the the Shattered Sun Offensive. You can’t find a single Alliance or Horde banner on the island.
Or at least that’s how it used to be before Chronicle inexplicably made it a faction thing.
I really really hate diving most raids by faction. Sometimes they work, like Horde doing Uldir, but most of the time it should be a united front - and that usually is how the game depicts them.