Ridley's Cohort said:
I am intrigued.
My gut instinct says this is not going to be a happy set of circumstances for our heroes, the Lich, or the Tarrasque.
Maybe someone should write a Soul Miscibility Table?
Well, it's kind of involved in this case. The lich is actually the ancestor of one of the PC's, who was killed by the Tarrasque about 2,000 years ago, but survived as a Jahvi (a powrful incorporeal undead from MM2) due to some magical preperations he had made (including nearly completing the ritual for Lichdom.)
I'm running in the Forgotten Realms, and in my campaign the Tarrasque is known as the Beast of Malar. It was created by Malar to slay the avatar of a giant deity who pissed him off (Ulutiu for those who know the realms). The ancestor was from a tribe of human barbarians who had traces of the avatar's blood running through their veins. He had also managed to steal a trace of the giant deities power, which attracted the Beast of Malar and killed him before he could make full use of it.
When he came back (by possessing the PC's sister, who had traveled to his crypt and awakened his spirit to learn sorcery), it awakened the Beast of Malar again, and it went after him. It couldn't catch him, because by this point the PC's sister was a high enough level sorcerer to fly and teleport, so he led the Beast around, leaving a trail of destruction. He told the PC's that he would release the sister and send the tarrasque away if they recovered this 50,000 gp gem (called the Heart of Malar) and a scroll that was kept with it. They did so, and he used the scroll to cast "Trap the Soul" on the gem and stood there while the Tarrasque ate him, which triggered the spell and trapped the tarrasque in the gem. His last act while possessing the sister was teleporting the gem to his crypt, where it served as his lich phylactery and he reformed his undead body.
The PC's aren't aware that the ancestor has re-incarnated himself as a full lich, or that the gem is being used as his phylactery. They have a prophecy that I handed out several months earlier that makes references to it though, so hopefully they will figure it out before they are high enough level to tangle with the lich, who will be a 16th or 18th level Sorcerer/Cleric Mystic Theurge when I stat him out.
The PC's are currently 7th-10th level, so hopefully that will be awhile (they have already inadvertently caused the creation of one dracolich by disrupting a ritual being used to ressurect an ancient ghost dragon. So they have that to look forward to as well.

)
If they try anything tricky like disposing of the phylactery by the "bag of holding in a hewards handy haversack" it just means the lich will come back as a Kaorti from the "Far Realms" as described in the Fiend Folio. Hehehe....
As for locating his phylactery, I think I have given the group enough clues to eventually figure out what his phylactery is, and as they have actually touched it themselves, they should be able to track it down with the appropriate magics when the time comes.