An Act Boss is the Boss that appears at the end of each Act. When you defeat it, you grant access to the following Act.
Depending on whether you’re playing in the Classic Diablo 2 or Diablo 2: Lord of Destruction, you have to slay Normal Diablo or Normal Baal respectively in order to unlock Nightmare difficulty. Further, if you kill Nightmare Diablo or Nightmare Baal, you will have access to the Hell difficulty.
The Act Bosses appear in the following order:
Act 1: Andariel
She’s also known as the Maiden of Anguish and she’s one of the Lesser Evils. Andariel was spawned from one of Tathmet’s (the ultimate first evil being) heads, same as all the Great Evils. Also, she ruled the Realm of Anguish in Hell.
According to Deckard Cain, she was the only female among the Evils. She assisted the Lesser Evils for many years, but eventually she lost her trust in their plans. Some years ago, she decided to assist Diablo during his revival in order to seize the Citadel of the Sightless Eye. Finally, brave heroes managed to slay her.
Andariel is the final Boss in Act 1 of Diablo 2 and is located in the 4th level of the Catacombs. Her weakness is the fire-based damage, so using fire spells and weapons is the most efficient way to take her down.
Act 2: Duriel
The Lord of Pain is the twin brother of Andariel and one of the 4 Lesser Evils. He was also spawned from one of Tathamet’s seven heads and he controlled the Realm of Pain in Hell. Before he was killed by heroes, Duriel was guarding the Tomb of Tal Rasha, which was the prison of Baal.
Duriel is the final Boss of Act 2. You can find him in Tal Rasha’s Chamber, which you can access by using the Horadric Staff on the orifice inside the real Tomb of Tal Rasha.
He’s one of the game’s hardest Bosses. This is because there are no exits inside the small room that the fight takes place. In addition, he has a rapid speed and a Holy Freeze Aura around him that it cannot be resisted.
Act 3: Mephisto
His real name is Dul’Mephistos and is also known as the Lord of Hatred. He’s one of the Prime Evils and he rules over the Realm of Hatred in the Burning Hells.
Mephisto’s evil was so pervasive, that even after his defeat and his entomb in a soulstone, his demonic essence flew slowly upwards into Travincal to corrupt the Zakarum priests.
You can find Mephisto at the 3rd level of the Durance of Hate. However, you have to slay a bunch of Bloodlords and some Council Member Bosses in order to battle him.
Mephisto has a combination of elemental attacks on his arsenal. For instance, he can use some Lightning skills such as Charged Bolt and a Poison Nova spell. Also, he can use some Cold spells like Frost Nova.
Act 4: Diablo
The Lord of Terror or Al’Diabalos is the youngest among the three Prime Evils. He rules the Realm of Terror in the Burning Hells. In addition, he’s the main antagonist and the character that gave his name to the Diablo series.
Diablo is the most dangerous of all Evils and his incapable of feeling fear. Although he had corrupted and possessed many humans, finally brave heroes trapped him and banished him into the Abyss twenty years ago.
You can counter Diablo in his Choas Sanctuary after you open all five seals. Three of the seals include a Unique Boss along with a group of minions that you have to kill as well.
The Lord of Terror can cast a set of powerful abilities and spells. The majority of them deal fire damage such as the Firestorm or Firewall. Moreover, he has a Cold Touch ability and his most effective attack deals Lighting damage. Thus, it’s better to max out at least your Fire and Lightning Resistances before you encounter him.
Act 5: Baal
His actual name is Tol’Baalos and he’s one of the Prime Evils. Baal is also known as the Lord of Destruction as the game’s expansion name indicates.
Baal was the most reckless and impudent of the Prime Evils. He was entombed at Horadim Tal Rasha after the Dark Exile. Many centuries later, Diablo freed Baal and then he corrupted the Worldstone. This had a destructive effect for the Barbarians that lived close to the Arreat Mount. Shortly afterward, the heroes killed Baal.
Baal is the final Boss of Act 5 in Diablo 2: Lord of Destruction. Before the battle, Ball summons groups of monsters from each Act. After you kill all the monsters, Ball goes into the Worldstone Chamber.
He has a wide range of deadly attacks and abilities. For example, he can cast Mana Rift, an attack that halves your mana points. Further, he has the ability to Teleport and to summon a copy of himself. Baal’s duplicate can cast almost all of the real Baal’s attacks.