You can encounter the Diablo Boss in Chaos Sanctuary (end of Act 4) after you have opened all five seals. The three of these seals include a Unique boss with a group of minions. In the upper seal, there is the Oblivion Knight, in the left seal stands the Grand Vizier of Chaos, and on the right side, there is the Infector of Souls. When you have slain all the Unique and mini-Bosses, an earthquake will shake the area, killing any remaining minions. Diablo is waiting close to the inner seal.
The Lord of Terror’s true name is Al’Diabalos and he’s the youngest among the Prime Evils. He’s the primary antagonist of the game and the Diablo series is named after him. He rules over the realm of Terror in the Burning Hells.
Diablo is the most dangerous and fearful of all Bosses and he cannot feel fear or terror. Although he had corrupted and possessed many mortals, finally his spirit was captured into a soulstone.
During the events of the Dark Exile, the four Lesser Evils banished the Prime Evils to the mortal realm. They spread chaos over the East for many years, but the Order of the Horadrim hunted them down.
The Horadrim utilized some artifacts (the soulstones) in order to contain the spirits of Baal, Mephisto, and Diablo. The Lord of Terror managed to escape to the West. However, the Horadrim found Diablo and contained his essence inside a soulstone. They buried the stone beneath an ancient Cathedral. There, Diablo waits for the right time for his revival.
This wasn’t Diablo’s end though. After 20 years of his death, Adria (his servant) was serving him in Sanctuary. She succeeded in gathering the seven Evil’s essences, which had been killed in the mortal realm. Diablo was planning to contain all seven essences into the Black Soulstone, so he can absorb all of them inside him. Adria formed an ally with a team of mortal heroes, including her daughter Leah and the Nephalem. Leah was going to be Diablo’s new “host”.
When the time was right, Adria bound Archangel Tyrael in arcane chains. Then, she used the dead guard’s corpses and drew a ritual circle that surrounded Leah. Adria Plunged the Black Soulstone into the flesh of Leah and the Lord of Terror was then born again. But the Seven Evils were now upon him, so he was more powerful than ever, the absolute Prime Evil.
The Lord of Terror has a wide range of powerful attacks and abilities:
Before Diablo starts casting his Red Lightning Hose attack, he makes a different animation from any other of his attacks. He will raise both hands into the air, giving you some seconds to dodge the attack.
Note that while evading the Lightning Hose attack, it’s better to run around the Boss rather than away from him. He turns slowly while he’s casting the attack so it’s quite easy to outrun it. Usually, Diablo uses this attack three times in a row. Thus, consider running in the opposite direction when the previous attack ends. In addition, Diablo charges at the players who are out of his melee range.
You should max out your Fire Resistance (75% or 95% with the proper items) and if it’s possible your Lightning Resistance too, especially if you are a melee class. Melee characters will have to receive Diablo’s melee hits, so they should always keep a Healing Potion handy. With a maxed out Fire resistance and probably some Fire absorption, you will easily survive through Diablo’s Fire Storm and other Fire spells. However, you should run away from the Red Lightning Hose attack as it deals extreme damage and will probably take you down.
Further, it’s important to encounter Diablo at an area where you’ll have enough space to run away from him. This also means that you will probably need stamina potions because you will have to run a lot in order to dodge Diablo’s attacks. If you are only walking, it’s almost impossible to evade a combination of his Cold Touch and Red Lightning Hose.
Ranged attackers should face fewer problems evading the Red Lightning Hose and Fire spells. They have to make sure that they don’t get charged by the Boss though. Moreover, you will have to retreat back to town a few times to refill your belt and inventory with healing potions. Keep in mind that you should open a Town Portal out of Diablo’s sight because he often casts Bone Prison around the Town Portals. He will have several seconds to kill you once you’re inside a Prison.
Diablo can’t follow you if you go down to one of the four halls. This makes the entrance and the seal rooms a safe place to cast Town portals or refill your belt with healing potions. Keep attacking the Boss until he finally dies. After a cool death animation, you can collect the loot and then continue to Harrogath.