In Diablo III, what was the name of the first expansion?

The first Diablo III expansion was Reaper of Souls.

Blizzard Entertainment released Reaper of Souls in 2014. Its story follows the angel of death, Malthael, who seizes the Black Soulstone and threatens Sanctuary. The expansion adds Act V, taking the player through locations including Westmarch and the Pandemonium Fortress.

Reaper of Souls introduced the Crusader class, a heavily armored holy warrior built around shields, weapons, and divine skills. It also raised the level cap from 60 to 70 and added Adventure Mode, Nephalem Rifts, and the Mystic artisan.

The title is sometimes mixed up with Lord of Destruction, which expanded Diablo II, or Rise of the Necromancer, a later Diablo III content pack. Reaper of Souls is the major expansion that reshaped Diablo III’s endgame and progression.