Death becomes and increasingly difficult endeavor to recover from. Each time a player character dies they must make a DC 10 (+ number of times previously killed + number of failed attempts to revive) constitution saving throw. This save is modified only by ‘anchors’.
A failed save means the spell used to revive the character has failed, and it was unsuccessful. Repeated castings will accomplish nothing, the only way to revive the character thereafter is through the use of a higher level spell, or a Wish or True Resurrection spell. Though even these have their limits.
Anchors are people, things, and unresolved business on the creatures home plane. Depending on the strength of those anchors (based on roleplaying), these add bonuses in the case of strong ties or an important quest, or potentially negatives, in the case of things the character wants to forget or leave.
Wish & True Resurrection
These potent abilities allow for remarkable power, enough to revive anyone with no chance of failure once, but only once. Efforts beyond that must succeed on the same saving throws as before.