Soul Mate on an Undead/Decaying target will trigger an infinite damage loop as long as you have any amount of lifesteal (one rank in necromancy) and your target does not have any remaining physical armor.
The healing done via soul mate is always "regular" healing, regardless of how you trigger the initial heal on your character. Normally Life Steal heals undead normally, but when transferred via soul mate it will damage them. Damaging their vitality this way steals yet more life, creating a loop. For some reason there is a minimum amount of healing/damage that can be done per tick, so even 5% lifesteal will perpetually damage a target instead of zeroing out (5% of 5% of 5% quickly rounds to 0, but the game always treats it as at least 1).
You can use this loop against non-undead/decaying enemies via Shakles of Pain (have undead party member cast Shackles, have second party member soul mate the undead), but in order to cast shackles of pain you still need to drop the enemy armor first, and you still need to be hitting vitality with soul mate, meaning your undead party member must have 0 armor as well
Because armor is WAY more significant than vitality, this infinite combo isn't really all that useful most of the game. Good for killing trolls, but anything else you can just chain CC once their armor is gone anyway.
