Underworld

Act VIII: Underworld is a multiplayer act consisting of seven bosses called the Eternals: Volcano, Megalith, Fungus, Vortex, Fatum, Arkhos and Hoaxen. After defeating Titan, Shadow ends up in the Underworld.

Underworld : Tier 1
Underworld features online raid-style battles which allow players to team up together to tackle the bosses. Each boss has a shield and the players must destroy it within a given time frame before they can challenge the boss. They can buy several items like small and medium charges of darkness to aid them and deal additional damage.

Eternals
Volcano - Eternal armed with Ornamental Sabers. First Boss

Megalith - Eternal armed with Coral Prickles. Second Boss

Fungus - Eternal armed with Two-handed Cudgel. Third Boss

Vortex - Eternal armed with Silver Glaive. Fourth Boss

Underworld : Tier 2
Underworld Tier 2 appears in version 1.9.21 and until now, the only bosses availible are Fatum, Arkhos and Hoaxen. Fatum is the only female boss in the underworld. You need 4 DAN and 1 key obtained by defeating Vortex to fight her. The next boss in underworld tier 2 is Arkhos who appears in version 1.9.22 and requires 5 DAN and 1 key obtained by defeating Fatum.

Eternals
Fatum - Eternal armed with Grim Scythe. Fifth Boss

Arkhos - Eternal armed with The Sting. Sixth Boss

Hoaxen - Eternal armed with Harrier Hooks. Seventh Boss

Underworld: Tier 3
Underworld Tier appears in version 1.9.23 and currently this tier hasn't got any bosses.

Eternals
Unknown (next version)

Trivia:

 * Soon after the launch of the multiplayer mode the strength of the bosses' shields was decreased. However the damage they dealt was instead increased.
 * Vortex, Fatum, Arkhos and Hoaxen are only eternals who require that the player reach a certain DAN before they can be challenged.