Help me design an artifact of doom

[Shinken]

In my next game, I want the characters to find an artifact of mass destruction. I was looking over the Pathfinder SRD, but couldn't find anything that really worked for me. The Castle of the Storm King kinda sorta fits, so I'm stealing some of its ideas.
The players would enter a cave looking for this weapon, then they would fight a guardian in a chamber and the artifact they were looking for would be only the key to the bigger artifact - the cave itself. Whoever touches the key would be possessed by it in some way. So the players would have to escape and the cave would become a flying fortress under command of whoever holds the key. That would be phase 2 of the campaign (World of Ruin for my FF6 ).
Now, the fortress needs to do something scary, but something the players can fight against. I'm considering allowing it to create a dark fog that works as enervation for everyone caught inside. This means you get wights, which creates panic, but also allows the players to help people out - it creates a lot of adventure possibilities.
I want the possession to do something badass as well. I'm thinking it might apply some template to the bearer (graveknight? I really don't know) and it will also be an intelligent item with a bunch of powers, with the purpose of replacing all life with undeath.

[Darkweave31]

Gramarie.You'll thank me later...