In Legion, I leveled my alt DK through class campaign and maybe I’m forgetting but was Bolvar/Lich King’s purpose ever revealed in raising 4 horseman, saying he had plans of his own for the death knights, etc.?
I feel like that it was never resolved. If it relates to Shadowlands, is it possible that he wasn’t truly defeated by Sylvannas and what he wanted was for her to open a way to Shadowlands?
Your first problem was assuming that anything beyond the broadest swaths of ideas are going forward between expansions anymore.
It really was to combat the Burning Legion so far as we know. Bolvar was no friend to the demons either. His methods are sketchy, but his goals aren’t bad.
The Ebon Blade didn’t want him to release the Scourge so as the pact of their alliance he would keep them back and they would act as his arm in the world. Reviving the Four Horsemen greatly boosted their power as a weapon against the Legion. Whatever his back up plans were in case our characters died or he lost control were never seen.
But for Shadowlands Bolvar didn’t really know that breaking the helm would do that. He thought she just wanted to wear it and control the Scourge. Bolvar was aware something was wrong in the Shadowlands, but his sight there is limited so he doesn’t know the full story.
Honestly for all the speculation that went around at the time about him having sinister intentions, I generally just assumed he was amassing powerful allies in the Ebon Blade because his hold on the Scourge was tenuous as long as he was the only thing standing between the world and someone coming to take his place and unleash an undead apocalypse.
Presumably the Ebon Blade - including the Four Horsemen - will have been fulfilling that purpose off-screen during the events of the reveal cinematic, holding back some sort of assault by Sylvanas’ loyalist forces while she slipped past them to personally confront Bolvar. And perhaps they’ll play a significant part in spearheading the efforts to quell the rampaging Scourge during the pre-expansion event.