Cain fears for the worst, for him, there is a possibility that maybe Albretch is Diablo's vessel. He will tell you to end this terror.
There isn't much to do, backtrack to level 15 and go down through the pentagram. You'll find yourself at level 16, Diablo's Lair. This lair is pretty messed up and filled with blood knights and advocates.
There is a complex system made up of four levers here. They are each located at the end of a large area, pretty similar to a maze. Pushing the levers until Diablo comes out. Have resistance against magic and fire ready and good armor equipped. Those demons, including Diablo himself, can kill you fast if you don't stay alert.
Diablo is pretty weak against almost all of the spells except Stone Curse – he cannot be paralyzed. However, a melee fight will be pretty tough since he is surrounded by minions. Keep drinking potions if you get close to death. Cast spells until he dies if you are a sorcerer, or attack from a safe distance if you are a rogue. However, if you are a warrior, try to use a vicious axe, as that may help a lot.
A good strategy is to call Diablo's attention and take him to a safe point away from his minions, use your strongest attacks before retreating if you take damage.
Diablo can cast Apocalypse, so beware.
After killing him, enjoy the ending cutscene.