The Prime Evil (Hell)
Once you fight him he will have 3 stages:
The first stage has 4 major special attacks, the first is a ground based fireball that explodes into a pool of fire that damages over time if your caught in it. The second is a flame circle attack. The second is a flame wall attack. The last one is bone prison, you must avoid this one or if you get caught in it he will teleport, grab you, drain your health, then smash you to the floor. Diablo also loves to teleport around in this phase.
To get to the second stage you must knock him down to less than two-thirds of his health. Then you will be sent into him realm of terror, you will fight 2 enemies in this stage, 1st is the Shadow of Diablo. The second is when you knock shadow Diablo's health one-third down. He will summon a shadow of your primary class that knows most of your abilities.
Once you defeat Shadow Diablo you will return back to the real Terror and fight his final form! He becomes more aggressive with his attacks and teleportation through his anger. Watch out for him Red Lightning Hose, does heavy damage if you get caught in it. You can take cover by one of the 2 heavenly crystals, or you can try to outrun it. Once he gets down to 5% health victory is yours!
Every class has their own abilities and strategies to beat him, but some great advice to take him out is using the health wells on the left and right of the level, and while fighting the Shadow Diablo, make sure you grab the health orbs he drops.
Good luck and have fun
Video is a demonstration of fighting Diablo done by Spo0ox