Merkuri said:
Demetri would become a GREAT on-going villain if he manages to survive this encounter. You might want to give him an escape route.
Our worst enemy is the one who used to be "one of us."
You can do a Darth Vader thing with him. Have him loyally serve the BBEG, being the one the PCs usually encounter rather than the BBEG himself. As it gets close to the final battle, give Demetri some opportunities to redeem himself, but let the character's actions heavily sway him, so essentially they determine whether he is at the last battle fighting with them or against them.
To build on this -- if you really want to be a bit Rat Bastardly -- here's what you do...
"Now my apprentice the time has come for you to prove your loyalty to me. Your former companions are approaching. Kill them. Kill them all."
What happens next is very much up in the air - but it will involve a fight, and a bloody one. Demetri is very much in Triel's thrall - addicted to the secrets he's been shown so far of magic, feeling betrayed and abandoned by his companions who left without him.
But... Demitri has a change of heart and turns on his seductress, who promptly and viscously wounds Demitri (but does not kill him) and then escapes at all cost.
Now, with a little luck, the players take pity on Demitri -- who will put up an appropriate show of contrition -- and allow him to rejoin the party as an NPC, or at least stay in touch with him as a usual contact and informant. Triel can now use Demitri as an inside agent, spying on the PCs' actions and whereabouts, and use him to feed them misinformation about whatever quest they happen to on.
Eventually, after he's thoroughly regained the trust of the party, he can lead them into a deadly trap or ambush. In the end it all falls to Demitri... He's in the position to save his old friends from certain death and destroy Triel, or he can let them die at the hands of his mistress. The PCs only really have a good chance of succeeding if Demitri helps them, he's wavering and struggling between two different loyalities and two different betrayals, and it's up to the PCs to convince him come back to their side... But how can they possibly trust him this time?