It's another Ritual. Theres quite a few in there, the Teleport and Negative Energy attack (to heal undead) are my favorites and using them deals ability damage. An easily cured trait.
Honest Pain on the other hand is permanent loss of two points of Constitution which means you need to be Undead and/or have access to Polymorph effects. It gives +2 to some social skills (bluff/diplo/handle) and prevents people from detecting your lies (lame). Since omitting certain details from the truth doesn't warrant a Bluff check and ignores Zone of Truth, this is only handy if the local government doesn't kill undead on sight, chained you to a wall, and interrogate you with direct Yes/No only questions. And by then you've screwed up a dozen ways to begin with. It sucks unless you are undead, then it's fine because it's free.