The first four legendary foes that you were supposed to eventually be able to fight in the Forbidden Arena. The purple one got replaced by the Queen of the 4th Age.

The Forbidden Arena, accessible from the Ship after consuming an Arena Ticket as well as from a previous Forbidden Arena, is barren except for the usual three Objectives and a podium with an open book on it near the spawn point. Interacting with this podium lets you choose one of four legendary foes to fight, each much more difficult than Caius, The Watcherdemon or Moloch, Ruler of the Spirit Realm:

When a legendary foe has been chosen, ominous drums will sound twice before the boss appears and the music changes to special boss battle music which will continue to play until you leave the planet.

The portals at the end of each Forbidden Arena lead to other Forbidden Arenas. You do not need to summon a legendary foe in order to use the end portals.