"As with positive-dominant planes, negative-dominant planes can be either minor or major. On minor negative-dominant planes, living creatures take 1d6 points of damage per round. At 0 hit points or lower, they crumble into ash.
Major negative-dominant planes are even more severe. Each round, those within must make a DC 25 Fortitude save or gain a negative level. A creature whose negative levels equal its current levels or Hit Dice is slain, becoming a wraith. "