long story short and SPOILER FREE !

i put the Key in, the coffin move a bit and nothing happen, it just stuck there like in this screen shot
http://img137.imageshack.us/img137/488/screenshot34j.jpg

i very very need to kill this dude, if anyone know how to fix this bug please let me know or help me find his NPC ID

I GOT IT ! NVM THAT

When you say the "key", did you find both skulls?