Malfeshnekor speaking with the PCs

[elef]

Hi all, I'd like to ask for some ideas about the Malfeshnekor encounter and specifically the beginning of it.
The party has cleared the first floor of Thistletop and next Thursday they are moving to the second and probably the third level if time allows it. What I'm planning for the Malfeshnekor encounter is once the party enter the room, Malf will be invisible and start talking to them (the meaning is to build some atmosphere before the encounter itself).
He will both taunt and measure them before the fighting starts. What are your suggestions on how he talks to them and what he says. I want to reveal some things about Thassilon but not too much. I'll probably mention the names of Alaznist and Karzug but not give out too many things about them. Mainly vague informations.
I'll also give them the chance to pinpoint him through perception checks. What should the DC be since he is talking and moving around?

[Askar Avari]

For the Perception check - Mal's Stealth is +10, and moving while invisible gives him +20, so his stealth check is 1d20 + 30. It's unlikely any PCs will be noticing him (unless they have scent).

That said, I might say they can hear something moving in the room at DC 20, but they can't locate where. That might give them the idea to bomb the room with a glitterdust or faerie fire.

And really, the only thing I'd have him talk about is cursing Karzoug for caging him for thousands of years, and threatening to devour everything between him and the big man himself. Seeing as he can't leave without Nualia's help, he may also curse the PCs if they've killed her on their way down.

[Nicolas Paradise]

While this is a unique angle He has been trapped in this room for literal millennia is insane angry and aggressive and even with the door wide open can't leave the room. Any and everything that isn't Nulia is an enemy as far as he is concerned not to mention that he is high CR enemy and can easily mess up an unprepared group.