Its Good to be Evil!
for player consideration (see the discussion):
Whenever a creature is subjected to 3 consecutive natural 20s on an attack, it is instantly slain. Exceptions: PCs and Major NPCs.
PCs and major NPCs subjected to a triple 20 will be ‘marked for death’ as if cursed. If the triple 20 happens at a time where instant death would be appropriate to the story, the character will instantly die, otherwise they will die at the next story-based opportunity.
PCs and major NPCs get a Fort Save to negate this effect equal to 10 plus the CR of the creature delivering the crit, at the time it is delivered. If the PC doesn’t instantly die, the effect can be removed magically (research may be required) if accomplished before it is fulfilled (i.e. the next story opportunity).
For PC death, a “story-based opportunity” is one that involves a major plot point, opponent, or other element of significance to the story arc. For major NPC death, it is a climactic or near-climactic moment, as judged by the DM.
A major NPC is a recurring, background, or leading NPC of significance to the campaign, such as the Baroness Forsym or the Emperor, but may include as-of-yet undiscovered NPCs. Fenril would not count.