Make it so Malzahar Malific Visons can only be placed on a silenced champions
Or make just it so you must be oppressed in some way, right now it is a no skill spell that does an insane amount of damage thanks to death fire and liandryies interaction and the way your trying to fix it doesn't make sense, don't make the void zone do less damage just add the thing Malzahar needed for a very long time, counterplay.
And to counter balance it you could give it a much larger range.
Also obviously it could still jump from minions onto you and it could freely be placed on non oppressed minions.