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As an example: Lets say I want to build something using Volatile Zombies as its main damage source. Right off the bat, my choices are Necromancer or Lich. Necromancer has obvious minion support and its existing defense options (armor, minion taunt, ward) don't clash with Volatile Zombie. Lich has some extra flat necrotic damage in its tree, a lot of cast speed potential, more movement speed. But, generally the main draw of Lich is Reaper Form and Death Seal. Reaper means we need a form of hp sustain and we can't Leech from the zombies, but they can give us hp when they die. Death Seal can sort of interact with them offensively by shredding necrotic resist or briefly buffing them if I can hit them with a death wave, but Death Seal disables ward, which is a very attractive aspect of zombies.

Lich is probably the suboptimal choice here, but the mobility of Reaper Form could be something you're really interested in.

Assuming we want to build a Lich version for some reason, we then look at the skills that could support Volatile Zombies to enhance it or cover its weaknesses.

The main thing to think about is if we're going to use Giant Zombies or not. Giant Zombies could be a significant damage increase and ward generation increase if we can sustain the mana. The primary ways we might do that are Rip Blood, Drain Life, or Mana Regeneration. If we go Rip Blood, we want to scale A LOT of cast speed. If we go Drain Life, we know we're sacrificing some smoothness/speed for infinite mana, if we go Mana Regeneration we're probably going to have some down time but we free up a skill slot in exchange for prefixes. Then we need to think about what we're using our skill slots on. Reaper Form and Zombies are obvious. Death Seal, Aura of Decay, Transplant, Bone Golem, Bone Curse, Rip Blood, and Drain Life are all options.