Wizardry: Labyrinth of Lost Souls

Story-related. Can be missed if you make the wrong choices.

On the final quest of the main storyline [Rescue King Diement V] you'll have to make two choices when talking to Shiin. To fight Tempestus Rex you have to choose "You are not to blame" and "You don't need to continue suffering". After that you'll come to a point where you talk to Gregor on B7F of Shiin's Dungeon (X:18, Y:16) after fighting the Hydra.

Choose not to believe him and continue on to B10F (X:10, Y:14) to fight this boss. He's very tough. If you're having problems I recommend using the Bishop's "Magic Wall" ability and spamming "Gas of Despair" with as many characters as possible. If you're lucky he won't use any kind of AoE before you can defeat him.

For more information on the quests you need to do and coordinates see Savior of the Empire.