if you have 25% of your mp and dark Side is up, you can use dark arts, thats it.

I cant tell if you are trolling... you cant use dark arts before learning dark side, because you learn dark side at lvl 30 and dark arts at lvl 45 AND dark arts requires you to have darkside turned on.

Heres abyssal Drain tooltip:
Deals unaspected damage with a potency of 120 to target and enemies near it.
Additional Effect: Increased enmity
Dark Arts Effect: Absorbs 100% of damage dealt as HP Dark Arts fades upon execution.