The trouble with death magic is that it tends to be very long lasting. Where sword blows, poisonings, and broken bones are physical wounds, the injuries sustained by the wielders of death magic are of a spiritual nature. Thus, hit points lost to death magic are permanently lost because a certain amount of the character's life energy has been stolen. The same is true for loss of attributes. For example, certain Undead drain Strength. A Character whose Strength is drained does not gain those lost points back simply by rest or by low-level healing invocations.
Undead most commonly wield death magic. Their touch can permanently drain hit points, spirit points, and sometimes even primary attributes. These injuries cannot be healed by conventional medical treatment or by normal healing invocations. More powerful intervention is required. The Priestly invocation Imbue with Life Energy, and the Wishsinger's Song of Life Magic are two examples of powers that can undo the effects of death magic - in all cases, a spiritual power will specifically say if it can undo Death Magic effects and damage. Most frequently, time is what cures these injuries. A character can recoup lost attributes and abilities when advancing in level (since the time to advance in level typically accounts for months or even years of the character’s life, it can be assumed a level’s worth of time is sufficient to recover from spiritual wounding.) Note that using a Wish can also restore losses suffered by Death Magic.
If a player character has hit points, spirit points, or any primary attribute drained to below zero, the character is slain and there is a 5% chance per hit die of the slayer that the character will reanimate as an Undead of hit dice equal to his level. The Undead character then becomes a monster, controlled by the Game Master. The newly created Undead will have 50% of his old memories intact, but his innate power will become a death magic variant, and his class related spiritual powers would cease functioning (for the exception of Call Upon Chosen Weapon, and even that power becomes twisted.)
Of course, some Undead have the power to Create Spawn. In this case, the rules for spontaneously resurrecting as an Undead are ignored, and the specific rules for creating spawn for that type of Undead are followed instead.