Diablo II: Resurrected

You've made it to the Chaos Sanctuary. You've destroyed Mephisto's Soulstone. Now it's time to tackle Big D himself. As the original final boss of the game, and the game's namesake, he is of course, not a pushover, so come prepared for a solid fight.

Preparing the Sanctuary.

Before fighting Diablo you have to unlock 5 seals. Three of the seals will unleash a unique enemy, and some minions, who must also be defeated before Diablo will spawn. The Chaos Sanctuary is one of the least randomly affected zones in the game so you can always know which seals will unlock an enemy, and which enemy, as well as where they are.

There are two seals on the west (left) wing, one (top/further) which will unleash the Grand Vizier of Chaos, a super unique Storm Caster, and minions.
There is one seal to the north (top) which will unleash Lord de Sais, a super unique Oblivion Knight and minions.
There are two seals in the east (right) wing, one (the bottom/closer) which will unleash Infector of Souls, a super unique Venom Lord

"Not even Death can save you from me."

Diablo has six different attacks he can choose from depending on range from his target and how the RNG goes. Most of them are fire based, so having high fire resistance is advisable.

A physical attack at melee range which applies a chill effect.
A firewall attack at range
A flame circle, which radiates out from Diablo
A firestorm attack which creeps along the ground from Diablo to his target in random paths
A charge attack for characters outside melee range

And by far the most deadly: Lightning Breath. This attack shoots a beam of lightning doom which Diablo can rotate as he targets you, though he will remain stationary while he performs this attack. The lightning attack is actually both lightning and physical and the total damage from this attack, if it hits you, is shocking. Lightning resist, damage resist, and blocking can all help, but it's just generally better to dodge it entirely, if you can.

Diablo also can summon a bone prison, which will immobilize the target until it is destroyed, though you can still attack/cast through it. If he imprisons you and then uses his Lightning Breath, I'm very sorry. He will also target open town portals to prevent you from returning to town. This can be avoided by opening a new portal if needed.

Diablo is also almost certainly going to be higher level than you, which means your % chance to hit with melee attacks is going to suffer. Diablo is Level 40 on Normal, 62 on Nightmare and 94 on Hell. Remember, % to hit is based, in part, on level difference. Those extra misses will add up and give Diablo more time to hit you with his own powerful attacks.

The fight plan is going to differ based on class, but this will likely just be a longer and tougher fight in a style that you're used to by now. The Chaos Sanctuary has room to kite, plenty of resources from the previous work to clear (e.g. corpses, potions, weapons/armor if you want metal golems, etc.) This allows you to be well-prepared for the tough fight.

Don't forget that you can dash back to town at any point. Have a portal open at all times so you can heal up, restock potions, resurrect your mercenary, or just catch a breather. Diablo will wait!

Nightmare and Beyond

Diablo on later difficulties scales up well, but gets a bit easier for melee characters, because otherwise the balance would suffer. The lightning breath attack hitbox is reduced in Nightmare, to exclude the area right in front of Diablo, allowing you to continue to melee him without getting shredded instantaneously. The hitbox is further reduced in Hell difficulty. Most classes will be well specialized by fighting Diablo in Nightmare and be able to handle him with their own style, but if you're not using a traditional build, this will probably be the end of the road.

Farming Diablo runs is still profitable and a good way to gain experience particularly when you are at/near the level of the zones, though other locations are in general going to be faster and more profitable.

Some final notes and possible glitches

If you kill a super unique, his minions should die. In previous versions of the game some of the Grand Vizier's minions would occasionally float into inaccessible locations making it impossible to progress. By having them die when the Grand Vizier dies, this is prevented.

If all super uniques are already dead when you open the fifth seal, all remaining enemies in the Chaos Sanctuary will die as the Diablo spawn begins. Any enemies loaded into memory will drop items, as if they'd been killed. This can be used to farm. If all five seals are open, killing the final super unique will begin the Diablo spawn, this kills other enemies but without farm potential.

The Venom Lords make very good Revive targets for the Necromancer because of their damage output vs. Diablo.

Diablo may target his bone cage at your town portal. If he does, in an extremely rare glitch, he may himself become trapped in it. He is programmed to not attack his own bone cage, meaning he won't attack until it is gone, giving you free reign.