Is there any useful necromancy spell to use along with the build?
Depends what you are going for. If you are going for a blood build yes...
If not then Chains is a great ability, Mosquito Swarm is decent.
But one skill you will want is Chloroform bottle as it will be your only source of magic damage and CC AND works for wizard assists as it does amazing Magic Armor damage.
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Decaying Touch is iffy! It is probably the most potent Necromancy spell or heck... Spell... In the current game due to the damage output of healing spells.
Heck it even, in theory I haven't tested it, works on undead and turns their zombie attribute into their greatest weakness (Healing AND Poison will kill them).
IF you have a team that can exploit it... I suggest picking it up. Otherwise, forget it.