Let's Read The Immortal Handbook Bestiary

[Khedrac]

Weirdly, the Sadim’s DR and Regeneration can be bypassed by weapons that were given to the PCs as gifts. I could easily see this turning into a no-win scenario, where the players can’t actually kill it.

This is the sort of symbolic thing which is great in a story or a myth, but is really not great when one is trying to make a mimimally balanced game.

This basically comes down to "do the PCs make the knowledge check to know this?"

It is the standard technique for dealing with something whose regeneration you cannot bypass right now - pummel it down to several thousand negative hit points and then find a way to kill it.
If they make the knowledge check then one party member says to the main damage dealer "here, I give you my dagger" and one coup de grace later the Sadim is dead and the dagger gets given back.
If they don't, it's either you spells that can kill by bypassing the regen mechanic or wander around with an unconsious creature until someone can tell them how to kill it.