I'm making a vampire anti-paladin as a fight in my game however I just noticed that undead use charisma for fort save but anti-paladin adds charisma to all of their saves, so do I add this twice then or just once?
I think the correct thing to do is add it twice.
You add it the first time as per the rules on undead.
The second time it is added as an anti-paladin class feature.
But first add some half-fey or something template so that the charisma modifier goes into the stratosphere! I want to see that fortitude save modifier in the +25-30 range!
It is added twice.