Cyclades: Hades - Rules of Play says:
The Cyclades: Hades expansion consists of four modules that can be used individually or combined in various ways. In the Cyclades base game, players fight for domination over archipelagos in ancient Greece. They make offerings to gods to secure their favors and summon mythological creatures. The Hades expansion allows players to add Hades and be able to build a Necropolis and summon undead armies when they gain favors of Hades.