After playing around with the spell, I realized that it wasn't returning spirit health at all any longer, and wasn't doing any damage for Undead backlash. Turns out a variable was getting hosed by a check that was added late in the development of the spell, after the spirit leeching had been tested. This bug was also preventing the backlash damage from Undead targets from actually taking place.

Both these features should be fixed. Leeching your target's spirit should now actually work, and casting on Undead targets will now be rather dangerous - so take care against those foes until you get more acquainted with it.

-GM Skralthaen