Ilambris is a Monster Set (Helm and Shoulder) available in light, medium or heavy armor. It drops in the Ilambris (unique) style.
The 1-piece set bonus adds 1096 Max Magicka.
2-piece set bonus: When you deal Flame or Shock damage, you have a 10% chance to summon a meteor shower of the damage type that deals 1170 damage to all enemies within 4 meters every 1 second for 5 seconds. Each effect can occur once every 8 seconds.
Ilambris is a very popular set with Magicka DPS players, especially Sorcerers who do a lot of Shock damage, although it also works well for casters who use fire staves, or Dragonknights who do a lot of Flame damage.
The Shoulder can be acquired by using an Undaunted key on Gilirion the Redbeard's chest. The Shoulder type, weight and trait are RNG based and can be from any dungeon this NPC gives Pledges for.
The Helm can be acquired from Crypt of Hearts I which is part of the base game and located in Rivenspire. The Helm is guaranteed to drop from the final boss in veteran mode only. The weight and trait are RNG based.
You can mix-and-match Monster Set shoulders and helms from different sets and add two 1-piece bonuses to your build, or wear a complete set to take advantage of the full 2-piece set bonus.