it will be raid and party bosses you will get quest from deckard cain leading you to kill raid bosses, diablo, mephisto, baal and bringing their soulstones back to cain, you will later in the quest line visit tristram, wirt, griswold, duriel, andariel, cow king and secret cow level (and maybe also see tyrael fight izual (and maybe join in the fight MAYBE JUST MAYBE))