starting this now. imho. the previous seasons don't deserve that high of praise. The story line is kindof fucked. and, alot of characters actions and motivations are just ?????
Viren especially. what even the fuck is this guys plan? for my king and country, kills his king. for humanity!, is the butt buddy of some evil mirror elf. Nothing this guy does or says makes any sense... dark magic! show never explained why dark magic is actually bad.
All magic users in Xadia are 'in tune' with the primal sources, and can use magic corresponding to it. Dark magic is only possible by killing a living thing and using that to fuel the magic, so its abhorent to all the other magic users (except maybe Aravos? Dunno).
That's at least how I viewed it.
As for Viren, his motivations are driven by conquering/eliminating the rest of Xadia and being on top of the food chain. When he couldn't take the throne by force, he conspired with one of the other kingdoms to make it happen. Tie that into his lust for power (black magic being addictive I assume?) and he saw the chance to become a god via the weird wormy critter.