Act IV Quest 3: Terror's End - Diablo 2
Quest Location: Chaos Sanctuary
Quest NPC: Tyrael
Quest Goal: Defeat Diablo
Reward: Unique item, Character Title, Access to Act V
Flag: Able to continue to Act V
You will automatically begin this quest the first time you enter Chaos Sanctuary and completing this quest will unlock the final Act in the game, Act V. In order to complete this quest technically all you need to do is defeat Diablo and then speak with Tyrael in town.
To begin the fight with Diablo you'll first need to activate all of the Seals in the Chaos Sanctuary and defeat all of the enemies they summon. There will always be 3 sets of Seals inside of Chaos Sanctuary (5 in total); two of them can be found in pairs and one Seal will always be alone.
Closed Seal on the left // Open Seal on the right.
If you've opened every Seal in the dungeon but Diablo still hasn't spawned for you it's likely that you missed one of the white spirit enemies that spawn with the Grand Vizier of Chaos. These enemies fly around the map and can go through walls/over lava and 'get stuck' over top terrain you can't navigate.
You'll know Diablo is spawning once the ground starts to shake followed by Diablo announcing himself to the zone. Diablo will always spawn in the central portion of The Chaos Sanctuary in the middle of the pentagram on the ground.
Diablo is one of the toughest fights in the game and definitely the toughest boss you've faced up until now. My recommendation is to go to town before you fight him and fill up on Healing Potions as well as Mana Potions. Put some in your inventory too or drop them in a convenient spot in Chaos Sanctuary if you're not good at using them out of your inventory quickly. When you pick them up, they'll automatically be added to your Potion Belt.
I'd also recommend you stack Lightning and Fire resist if you're struggling as they'll make this fight a whole lot easier. Diablo's primary damaging ability is Lightning and Fire is his second most commonly used element. Below I have put together a list of all his abilities so you know everything he can throw at you.
Charge: Diablo will sprint quickly across the floor on all four like an animal.
Cold Touch: A regular melee attack with a chilling effect.
Bone Prison: Diablo will cast this around Town Portals that are too close to him.
Firestorm: Similar to Fire Wall but instead of one straight line it is many erratic lines.
Fire Nova: A ring of fire shoots out from Diablo.
Fire Wall: Same as the Sorceress skill of the same name.
Red Lightning Beam: Diablo will cast a beam of red lightning that shoots out and is similar to a hose. He'll turn more slowly when using this ability and it has an insanely far reach. It's highly recommended you avoid this ability as it hurts a ton and is by far his strongest ability. Stacking Lightning resist helps a lot for this too.
A big tip that I can give to you is on Nightmare and Hell you can actually avoid the Lightning Hose ability by standing close to Diablo. On Nightmare you have to be right up in Diablo's face in order to avoid it but on Hell you can stand relatively close to Diablo and avoid it easily.
The way this ability works is on each difficulty there is a bigger gap in front of Diablo and where the Lightning Hose begins. On Normal, usually the Lightning Hose comes right out of Diablo's model. That is to say it's attached to him when he fires it. On Nightmare there is a small gap in front of him and where the attack begins. On Hell that gap is noticeably larger.
When I fight Diablo I typically try to keep him ontop of the pentagram he spawns on if I am ranged, if I am melee I typically pull him away from that location into a wide open area. If you have some sort of ability that can slow Diablo, like a Blizzard Sorceress, you'll be able to use the pentagram he spawns on to block direct movement to you and to 'path bug' him for lack of better words.
Chaos Sanctuary and Diablo himself are one of the most farmed locations in the game for loot as well as one of the better locations you can level up at. For more information about all of these things, follow the links below to my other Chaos Sanctuary guides.