The third and final quest of Act IV can be activated by talking to Tyrael after completing Hell's Forge:
|The time has come to hunt down and destroy Diablo, himself.|
But beware, the Lord of Terror is not to be underestimated. He single-handedly destroyed the town of Tristram and corrupted the last noble hero who tried to stop him.
This time, you must defeat him for good. Only by destroying the Soulstone which he carries will his spirit be banished forever.
Good luck! Though this be our darkest hour, it may yet be your greatest moment.
|Find Diablo in his Sanctuary.|
If you talk to Deckard Cain before completing this quest:
|You don't have time to dally about here!|
Diablo awaits you in Hell. Remember... Diablo's greatest weapon against you is Terror.
Don't give in to your fears. Resist his power and put an end to him for good!
|Proceed, hero, into Terror's lair.|
Know that Diablo's innermost sanctum is hidden by five seals.
Only by opening each of these seals can you clear your way to the final battle.
The Chaos Sanctuary is randomly populated by cursing Oblivion Knights with Doom Knights, mana-draining Storm Casters, and fire-breathing Venom Lords. Since v1.13, Oblivion Knights can no longer cast Iron Maiden, making it considerably less dangerous for melee characters dependent on physical attack damage, or Lower Resist (this appears to have been replaced by a non-functioning Decrepify cast at range): they always cast Decrepify on enemies within melee range (which is fully-functional).
The giant pentagram at the heart of the Sanctuary, where Diablo will spawn, is always to the northeast. However, he doesn't do so until you've opened all five seals in the three remaining arms of the Sanctuary, and killed the three Super Unique monsters that spawn as a result of doing so.
Once all five seals are opened and all three Super Unique monsters killed, any remaining monsters in the Sanctuary are also killed or removed before Diablo spawns (although you gain no experience): this can be exploited if you're finding one of the Super Unique monsters particularly troublesome, as after opening all five seals and killing the other two, you can focus on it, and all its minions and other enemies nearby will be killed when it dies.
Grand Vizier of Chaos
Two of the five seals can be found in the northwest arm of the Sanctuary: if it forks, the seal spawning the Grand Vizier of Chaos and minions is to the left; if it turns right, the same seal is still to the left, on the corner rather than at the end. The Grand Vizier is normally the least dangerous of the three.
Lord De Seis
There's only one seal in the northeast arm of the Sanctuary, spawning Lord De Seis and his Doom Knight minions around the bend behind you. Although they're far enough away not to be any immediate danger, create a portal before approaching with extreme caution: Extra Strong and Fanaticism is already a bad combination, made even worse by Lord De Seis' ability to curse you with Amplify Damage or Decrepify, both of which significantly reduce physical resistance.
If you can, get past them so that you have more room to maneuver in the body of the Sanctuary (although if he spawns Extra Fast in Nightmare or Hell, it may be more prudent to retreat via a portal and return via the waypoint in the River of Flame). Do not take on many Doom Knights at a time, particularly if they're still within radius of Fanaticism, unless you're very confident you can survive, due to significant life or damage reduction, or being able to prevent their attacks.
Infector of Souls
Two of the five seals can be found in the southeast arm of the Sanctuary: if its straight then the first seal, on the right, spawns the Infector of Souls and its minions; if it turns around to the right, then the spawning seal is at the end. The latter is much worse, since between the other seal and the size and speed of the Infector and its minions, it's extremely likely that you'll be cut off from the body of the Sanctuary unless you can Leap or Teleport: create a portal before engaging, and retreat and return via the waypoint in the River of Flame if you're having trouble enduring their assault. It may even be necessary to retreat as far as the southwest arm of the Sanctuary through which you entered, since its paths can be used as choke points to prevent you from being swarmed.
Not even death can save you from me!
Once all five seals have been opened and all three Super Unique monsters killed, then all remaining monsters in the Sanctuary are killed or removed (although you gain no experience from this). The screen shakes as Diablo finally makes his appearance at the center of the Sanctuary!.
You should create a portal, but make sure that you do so far enough away from Diablo that he isn't aware of it, otherwise he's likely to cast a bone prison on it which needs to be destroyed before you can escape (unless you can Leap or Teleport). When he first sees you, Diablo is likely to charge at you, and will do so whenever you aren't in melee range, but this is slow enough that it's easily avoided by moving. In melee range he has a 'cold' attack which actually inflicts magic damage, although it still inflicts cold length to slow you down unless you Cannot be Frozen.
However, his most damaging attacks inflict fire, lightning and physical damage. Aside from a fire nova (which can be avoided by correctly positioned casters and ranged attackers) and a seldom-used firewall, one of his most deadly attacks is a firestorm inflicting both fire and physical damage (so it can be blocked with a shield): he raises both arms to cast it, so if it's causing you trouble you can quickly move perpendicular to its course to avoid the worst of it.
The second of his most deadly attacks is a lightning inferno inflicting both lightning and physical damage (so once again, it can be blocked with a shield): he extends both arms in front of him to use it, and it can be avoided by running in a circle around him until he stops. Strangely, while even melee attackers may want to do this in Normal, in Nightmare it won't actually hit them if they stand close enough to Diablo, and in Hell the area of safety is even greater. However, if you are being hit by it, make sure you're not standing adjacent to another target: each inferno can hit both of you twice as often if you do so.
Like all Evils, Diablo inflicts 400% damage on any pets, but unlike those previously faced he's much more capable of targeting and destroying many at once. To protect any Necroskeletons as much as possible, a summoning Necromancer should use a combination of a Clay Golem's Slows Target and his own Decrepify to significantly reduce the frequency of Diablo's attacks so that his pets have as much time as possible to regenerate any damage that's inflicted. It may also help to summon some Revived, particularly Urdars from the River of Flame in Nightmare and Hell (since they have 15% Chance of Crushing Blow in Nightmare, and 25% chance in Hell).
Diablo is never immune to any damage type and doesn't regenerate (although he may reset if you spend far too long back in the Pandemonium Fortress), so any character should eventually be able to kill him.
As Diablo dies, a message appears in the middle of the screen, counting down: 90 Seconds Until Game Ends. If you return to the Pandemonium Fortress to talk to Tyrael and Deckard Cain:
|Praise be to the Light! You have accomplished the impossible!
Diablo and Mephisto have been banished back into the Black Abyss that spawned them and the corrupted Soulstones are no more.
|I knew there was great potential in you, my friend. You've done a fantastic job.|
Though my ancestors often struggled against the Three Evils and their minions, I've always lived a shut-in, scholarly life. I'm glad that my wisdom aided you.
Now, I wish to leave this place. Though Heaven's Gates are a marvel to behold, I hope I won't have to see them again for many, many years.
|You've done well, hero. For now, you should rejoice.|
Once the countdown ends or you save and exit, a screen appears to congratulate you for vanquishing Diablo, and inform you that your character will henceforth be known by a title determined by the difficulty just completed, character's sex, and mode played:
|Standard||Sir or Dame||Lord or Lady||Baron(ess)|
|Hardcore||Count(ess)||Duke or Duchess||King or Queen|
You can now enter the Secret Cow Level for that difficulty.
If you return to the Pandemonium Fortress to talk to Deckard Cain, a line of dialog is added:
|Please talk to Tyrael about leaving this place now!|
You need to talk to Tyrael, whose last line of dialog is replaced by the following:
|However, while you were fighting here, Baal remained behind in the mortal realm, building an army of hellish minions. Now, Baal's army is searching for the Worldstone, the ancient source of all the Soulstones and their power, while leaving behind a wake of destruction. They have forged deeply into the Barbarian homelands, heading directly for the summit of Mount Arreat!|
Baal knows, mortal hero! That is the very site of the blessed Worldstone!
Now, enter the portal I have opened for you. It will take you to the Barbarian city of Harrogath, the last bastion of Order on the slopes of Arreat.
Enter the portal to begin Act V...