Death from massive damage rules question

[Otterscrubber]

I know that if you take more than 50 pts of damage you have to make a fort save or die, but I could not find the rules on how to determine the DC for that fort save. Can anyone point out what I am obviously missing?

 

[Crothian]

The DC is 15

 

[shilsen]

What Crothian said. It's on pg.129 of the PHB.