Is there a feat for udnead to use CHA instead of CON for HP bonus?

[Sqwonk]

Is there a feat or rule that lets undead use their CHA rather than there 0 CON for HP bonuses?

 

[IcyCool]

I think it's a creature ability, not a feat. Check Libris Mortis.

 

[Scharlata]

I think it's a creature ability, not a feat. Check Libris Mortis.

Yupp! It's called Unholy Toughness (Ex), but it's from the Monster Manual III [see page 17, Boneclaw]. The formula is: Cha modifier x HD = bonus hp.

Note that there is also Unholy Grace (Su) in Libris Mortis to add the Cha bonus on all STs and as a deflection bonus to AC. [see page 84, Angel of Decay]

Enjoy!