I am told by the Blades that to be with them and not against them I need to kill Paathurnax who is up at his lair at the "Throad of the World".
I tried to do battle with him and he will not fight. How to get him to fight and can he be killed?

you have to go through the conversation with him first....

I went through the conversation with him.

The whole "Blades want you dead" thing?

Yeah.

It didn't work.

So I'm curious about this too, because I want to help the Blades and I can't do it without killing the big P.

I would prefer not to have to try and kill Paarthurnax....I kinda think he is pretty cool. But I also want to join the blades later on. If I don't kill him I am now an enemy of the blades??

If you have comppeted the main quest he is essential, and thus unkillable. You don't miss out on much though.

Edit _ Not kiling him makes the blades not talk to you, nothing more, killing him makes the graybeards hate you. I chose to let him live, the rebuilding the blades thing isn't much.

Not true. I completed the main quest and killed him after I came back from Sovngarde. This is what I did:

I attacked big P before talking to him so his health was down to nothing (but he won't die, and he won't attack). Initiated conversation and went through the dialog. After we finish talking, big P starts to fly away (after which I think he's gone for good). I then used dragonrend to bring him back down, swiped him once, and got one dead dragon. My blades quest updated and Delplhine is my buddy.

Give that a shot.

Attack him until he flies off the dragon shrine, then you can kill him.
He's immortal as long as he sits on that.

spoiler warning etc.



If you want to join the Blades without killing Paarthurnax, your limited window of opportunity is after completing Alduin's Wall (where Delphine asks you to start gathering recruits) until the beginning of Alduin's Bane. As soon as you read the Elder Scroll (first step of Alduin's Bane), you have to kill Paarthurnax. So, go as far as you want with the Blades before that point. It just becomes a finite dragon killing loop until you run out of locations to discover.