[PF] Losing all HD

[Renen]

What happens if you lose all of your HD due to something like Oracle's 'Awesome Display'?

[Psyren]

Awesome Display doesn't deduct the character's HD, it merely lets you (the Oracle) treat them as though their HD were lower to adjudicate specific effects (Patterns in this case.)

As a matter of fact, unlike 3.5, there is no way to actually lose HD in Pathfinder. Even Permanent Negative Levels just stay layered on top of your regular levels/HD, and never convert to actual HD/level loss.