He Came To Corrupt Them - Then Became Their Responsible Adult

"Infiltrate the heroes, corrupt them from within.
By dawn, the kingdom will...
Does anyone know which way is north?
The moss points upward.
This may take longer than expected."
He came in black with crimson trim, with poison rings and candles dim.
He forged a smile, concealed his name, and joined the fools to spoil their game.