Hi! I wanted the illusion spell tree to have some health damaging spells so I could fully rely on it. I was against a Draugr and didn't know what I'd do if I didn't have a 2 handed sword follower.
All spells have a very slow leveling effect, your leveling should mainly come from the original spells. The health damaging spells are pretty weak but should be enough to take down a dragon if you're patient unless you haven't set the difficulty to expert+. I tried to keep it balanced by making them cost quite a lot. The invisiblity spell isn't connected to any perk so it will always cost a lot to cast without enchantments.
Here's what I've made so far:

Saddenifier: Concentration spell. Drains health.
Description:The target has it's happy thoughts(should it be memories?) drained.

Depressing Thoughts: Fire and forget spell. Health damage and stagger.
Description: Depressing thoughts that causes X amount of damage for X time.

Concentrated Invisibility:Concentration spell. Invisiblilty, muffle and fortify sneak.
Description: Caster is invisible for as long as the hold the spell. Caster moves Silently.

I'm not very good at descriptions. Any help with that would be nice. If you have any other ideas for illusion spells, let me know and I'll try to make it.

Thanks!