You can't get rid of Rain of Blood as it serves two purposes.
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The other is it is by far overpowered... Dealing any percent of your HP as damage is absolutely devastating. 20% of your health can be 60 damage or more... More then AP skills typically do.
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So here is what Necromancy currently does
Necromancy is a skill set made to synergize mostly with frontline attackers, most notably Leechers but it isn't a requirement. It combos also with Air (and Earth/Water to a lesser extent). It has two main purposes, to sustain your frontline warriors indirectly (more set up based) and to deny the enemy the same through powerful CC-options. All the while being focused more on physical attack (Earth being the only int skillset of mixed types).
So you need to think of it on those terms.
1) It sets up healing and buffs through set ups or as secondary effects, often through damage.
--OR it protects the one who uses the skill, while still targeting the enemy.
2) It denies and reverses enemy advantages such as healing.
--OR It punishes the enemy, instead of shutting them down
3) It deals more damage against stripped enemies rather than through combos
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Well, how about a skill that doesn't cripple the enemy but instead deals damage if they move and how far they move? AND allow it to combo off of Teleport.
I'll say I agree Rain of Blood doesn't need to go, though maybe it could do a small amount of damage to enemies. Could be a separate skill like the OP's suggestion (Soul Punisher).
Dealing damage as a percent of HP obviously will take some balancing --though 20% being 60 damage would mean a 300 health character, which would mean almost pure investment in CON until about level 8 I think. Still, 10-15% might be better. A pure intelligence mage can easily do that kind of damage with multiple abilities, though obviously they aren't gaining damage AND health like a CON mage would, so probably the necro damage should be less.
But doing damage as a percent of current health is an interesting dynamic, especially if that ability heals, since it will deal more damage when it's less useful for healing. I'm not sure if I like that or not, though, since it makes it kind of seem like the skill is being wasted a little no matter when you use it.
I'm not sure if necromancy quite as a clear identity yet with as few of skills as it has, but I think you're mostly correct in your assessment, as a sort of debuff/soft cc/reversal/self-sustaining tree, so damage should definitely be on the lower side with all the utility it grants. I do feel like it could be somewhat more viable as a primary skill tree for a character, though, which a bit more damage and more debuffs and reversal skills could allow for. It could sort of be the tank skilltree, too.
I like the idea of a skill that makes characters take damage for moving, too. Perhaps it even actually grants a movement speed buff, to make it possible to go either way to be used a buff or a debuff.
