KiloGex
First Post
If I were you, since your error resulted in a TPK, I'd retcon the whole encounter and let the hidden PC try to disrupt the enemy's black tentacles with his lightning bolt. It seems only fair.
Rather than simply calling "My B" and redoing the entire encounter (which will give the players a heads up now that they now the BBEG's strategy and abilities), why not just say "Look, I made a mistake so I won't kill your characters completely." Instead, chose one of their deities and pull the divine champion card - "I see great things in your future, and your task is not yet complete" type thing. Drop them back into the world, with a compulsion to complete their task. Give them the -1 level for resurrection so that they don't just march right back to the BBEG and kick his butt, but maybe boost their XP until they return to the next level.