THE ENDING MOVIE
The end movie is built on a series of blocks. Which blocks are added to the movie depends upon what you did, or didnt do, in the course of the entire game. For a breakdown of those blocks, I can do no better than to refer to (and supplement when I can) JaggedJims walkthrough.
Zuggtmoy is Banished: Banish her either by taking the gems from her silver throne, or waiting out the 4 days after the Orb of Golden Death has been destroyed.
Zuggtmoy is Killed: Slaying her in combat.
Zuggtmoy Surrenders: She stays sealed up in her Sanctuary for a while, but it is not long before she falls back on her scheming.
Zuggtmoy is Enthralled: I wasnt able to test this out. Since the Official Guide indicates that it is possible, I assume that theres a separate block for this.
Lolth Wants Vengenace: Since you killed her cleric, Lareth, in the Moathouse.
Prince Thrommel is Freed: If you freed him from the Temple of Elemental Evil, Level 3. He and Princess Jolene wed, and the kingdoms of Fuyondy and Veluna unite.
Hommlet Prospers: Kill or ban Zuggtmoy.
Hommlet is Destroyed: Join allegiances with Hedrack and become initiated into the Greater Temple.
Nulb is Destroyed: Kill or ban Zuggtmoy.
Nulb Prospers: Join allegiances with Hedrack and become initiated into the Greater Temple.
Kelno Escapes: Kill or ban Zuggtmoy and leave Kelno alive. He escapes only to become a beggar.
Belsornig Escapes: Kill or ban Zuggtmoy and leave Belsornig alive. He escapes to renew his scheming and plotting.
Romag Escapes: Kill or ban Zuggtmoy and leave Romag alive. He is never seen again.
Alremm Escapes: Kill or ban Zuggtmoy and leave Alremm alive. He escapes only to become a raving lunatic.
The Temple of Elemental Evil Disperses: Kill or ban Zuggtmoy and kill all four of the High Clerics. With the leadership of the Temple eradicated, its followers soon scatter.
Prince Thrommel Aided Your Party: If Prince Thrommel was in your party to help you kill Zuggtmoy.
You and Bertram: If Bertram was in your party, he and the character who picked him up ride off into the sunset so to speak.
Marriage to Meleny: If Meleny is in your party, one of your characters marries her. He and Meleny then have many grand adventures together afterwards.
Paida and Valden United: If you rescued Paida and brought her back to Valden, they live happily ever after.
Prince Thrommel Becomes a Vampire: To get this ending, you must finish the game through one of the good paths, which is to say ending the Temple's threat instead of swearing fealty to Hedrack. Also, you must not come into any contact with Prince Thrommel and leave him as a prisoner within the Temple. The result: Prince Thrommel becomes a vampire and strikes fear into the local populace. His sword becomes lost to the sands of time. Thanks to Matt Rauscher for pointing this out to us!