In Diablo 1 King Leorics son Prince Albrecht is kidnapped, driving the king mad, and used to give Diablo a human form by plunging the soul stone in to him, but he is not strong enough to let Diablo escape the dungeon. The warrior is his brother Prince Aidan and during his decent down in to the dungeon he is corrupted in to believing that the only way to control Diablo is to plunge the soul stone in to him self, but this is just a trick by Diablo to get a host that is strong enough for him to get out of the dungeon.
If you want more of the backstory i sugest watching this 3 part video: Pre Diablo 1, Diablo 1 and Diablo 2