When do I die?

A PC will die when his hit points reach an amount equal to half his total hit points expressed as a negative number.

Example: if PC1 has a total of 80 hit points, he will die when he reaches -40 hit points.

Also, as soon as a PC drops to negative hit points, he will have to roll a Fortitude DC15 save every round he stays at negative hit points. If he fails 3 saves, he will die. A save roll of 30 or more will "stabilize" the PC, and he does not have to save anymore, unless he suffers more damage, in which case the need to save resumes.

Example: PC1 has total 80 hit points, and Fort save +12. He drops to -10 hit points, meaning he's not dead, but will now need to roll Death Saving Throws every round.
One the first round he rolls a 2, for a total of 2+12=14, failed!
On the second round he rolls a 1, which is always an automatic fail... he's now one step away from death.
On the third round he rolls a 10, for a total of 10+12=25, meaning he succeeds, but will still need to roll next round.
On the fourth round he rolls a 19, for a total of 12+19=31... he's now stabilized, and does not need to roll saves anymore. He's still at -10 hit points.

If a PC at negative hit points is healed by magic, his hit points automatically reset at ZERO, and then the magic healing applies.

Example: PC1 is at -10 hit points. PC2 casts Cure Moderate Wounds on PC1, healing 20 hit points. PC1 now has 20 hit points.