A cool ability I think one of the darker fantasy specs could use would be something similar to putting a soul back into a body for a short period of time.
Soul Tether: 10 minute CD, castable in combat.
Resurrect an ally for 2 (adjustable)minutes, during which they receive a buff (for mechanical purposes) which decreases their damage and healing done by 10%. The amount of damage and healing done is further decreased over the duration of the buff, up to 50% when the buff expires. They are instantly slain upon expiration (this second death wouldnt count in M+ times, I would think.
It would add another panic button certain classes could have as a battle rez without lowering the value of the classes which do have one. The reason the ability would be a buff instead of a debuff is so they can remove it when they want, instantly slaying them on the spot to be ressed, if they finished killing a certain pack or boss.