This cost does not decrease your level even if it is greater than the amount of experience you've earned. It also does not remain as an experience debt.
If you've died: Edit
If your character has died he cannot recall anything that happened in the fugue or his encounter with Myrkul, or any players there. Sending in character tells from the fugue plane is also frowned upon as meta-gaming. In the event that your character met his demise at the hands of another player in a PvP situation the no-memory rule extends further than the stay in the Fugue. It means that your character cannot recall or remember how he died and what immediate events led up to his death.
Permanent Death: Edit
Baldur's Gate does not have forced permanent death for characters. Dungeon Masters will never pursue such a thing unless a player consents. In that case it may be possible for the Dungeon Masters to organize some events leading to the character's permanent demise / retirement. At this point you can then delete the retired character via the RCR NPC in the nexus.