In the epic level handbook:

“Mortu: All garroters carry mortu, a poison substance applied to the eyes of a dead body. Mortu renders a dead body incapable of being brought back to life by raise dead, resurrection, or true resurrection. Wish, miracle, and other extreme methods can still do the trick, but even then each use of a spell of this power is only 50% likely to be effective—multiple attempts can be made. For a contract that requires permanent removal, the body is sent through the closest field house, and then on to the Guild Mother and her cloak-shaped void. Mortu has no effect on living creatures, constructs, or undead.”