He had a whole part of his story about how he had void magic, but opted not to make use of it, since he favored the Light. What happened with that part of his story?
Maybe he could have a line where the Light considers him impure after being Lich Kinged, which leaves him with only the void. Or maybe we’ll just not mention that part of his story anymore.
Just hasn’t been relevant. It’s very likely that this will be a major arc for Anduin once we venture into the void arc of the game. The light has shown incredible favoritism to him so that should be interesting.
He used void stuff to torture one of the undead night elves for information on where Sylvanas went
Even he was surprised by what he used and he doesn’t even know what he used!
Jaina seemed worried by the Purple Energy being used herself despite the Book not saying what the Energy is!
Sylvanas used an unknown Purple Energy that has a distinct look different from Void Magic herself and no one knows what it was despite Thrall recognizing the Rainbow Domination Death Magic used by the Mawsworn as the same Death Magic used by the Shadowmoon Clan!
If the Purple Magics are the same no one knows what it is as neither Zovaal(who in combat combines the Rainbow Domination Death Magic used by Frostmourne, Forsworn and Mawsworn with the Blue Domination Death Magic used by the Drust and the Helm of Domination) nor Sylvanas bothered to study it despite empowering it with Anima!
Zovaal’s research ironically enough focused on Frostmourne’s Rainbow Domination Magic gifting it to Mawsworn(and according to https://wowpedia.fandom.com/wiki/Her_Will,_Inflicted#Notes extracted it from Uther to create Forsworn) with the only research into the Helm of Domination being for his personal use and to create the thing. The Drust studied the Helm of Domination’s Blue Death Magic further than Zovaal and probably got it from somewhere other than the Helm since the Primus didn’t disappear and have his plans for the thing taken until after Draka joined the Shadowlands.