Illuster is the center of power for the Arbiter's Guild and Immortal Council. It has the typical services offered by cities (a Shop and Guild), but also has a story encounter that takes place later in the game.
Story Encounter[]
Level range: 31-38
Rewards Demon Leather, Gold Thread, and Guardian Sword
This fight pits you against various demons, including an Aeoth. 
Holy damage is recommended, as is teleportation or Jet Boots in order to more easily get units to the ramparts. You will be accompanied by "Guardsman," a Templar/Duelist with a Guardian Sword equipped. If kept alive at the end of the encounter, you will receive an additional 100 AP. He has very high defenses and is unlikely to get himself killed, so you shouldn't need to focus too much on keeping him alive. Since you are up against Malcubi and Archafflictors, 
Barrier and/or immunity to Debuffs is also helpful.