Can someone explain level draining?

[KillianHawkeye]

What do they need to succeed on the fort save? And do they have to do it for each level, or all at once?

You make a seperate save for each negative level. The DC is 21 for a standard succubus, but if Sabine has additional levels or a higher Charisma score then it will be higher.

And they can always be restored, right? Is it expensive?

A greater restoration spell will return all lost levels, but only ones that have been lost within one week per level of the guy casting it (minimum 15 weeks since it's a 7th level spell). It doesn't cost any money, but the caster must sacrifice 500 XP.

Does a player typically encounter level draining enemies before they have the ability to remove it?

I have no idea. If you have a Cleric on hand, you can remove all negative levels from a person before they become actual level loss by keeping restoration prepared, which you can cast by 7th level. There shouldn't be too many level draining enemies before that.

Also, if you learned a feat on your current level, you lose it? Like, you lose anything that you gained, just as if you never earned the level?

Only if your negative level becomes permanent level loss. At that point, you basically need to recalculate your character's stats at your lower level.

Sorry for the slew of questions. Should be pretty easy for a learned player to answer though.

No problem, although there's this handy website where you can find this sort of information on your own.