Your legion fights on its own. You pick a formation of up to four heroes out of five, gear them up, and they handle the rest, wave after wave. Close the game and they keep going: the offline progress is worked out on the server.

What you decide is who you level, what you put on them, which pets you bring and when you push the next boss.

Five acts, five bosses. Each one fights differently. The Ancient Ent and the Magma Lord have a break bar. Break it and the boss is stunned and takes double damage. The Ent and the Void Reaper hit harder at half health. The Swamp Hydra regenerates unless it is burning. The Archnecromancer opens with a bone shield. The Magma Lord switches between heat and cold. Each act boss asks for that act's key. The stage before the boss always gives you the first one. The rest drop as you play. A key is only used up if you win. Keys are not sold for real money. Players trade them for gold at the auction house.

Five difficulties: Normal, Hard, Nightmare, Hell and Madness. The same five acts, with tougher enemies.

Seven gear rarities: from common to mythic. Rerollable affixes, elemental runes and forge stones.

The Abyss: a dungeon that builds itself. In the level 100 Abyss, the final boss can drop mythics. Mythic is the top rarity, with five affixes. The Forge of the Well also crafts them from Mythic Cores. Those cores drop in the Abyss itself. Outside it, fusing ten uniques in the forge can give a mythic. It happens one time in two hundred.

Forty pets: each with its own bonus, and twenty levels to raise it.

Asynchronous PvP: your formation against another player's, with leagues and seasons. The fight is resolved on the server. You watch it as a replay.

World boss: its health is set against the whole server hitting it at once, not against your team. Rankings by player and by guild.

City and guilds: your own avatar, an isometric plaza and an auction house.