Icelandis, otherwise known as Floor 4, is achieved by killing Droggrel the Golem Master, the Floor 3 Boss. You get Icelandis by getting 20 percent (or more) of the hits on Droggrel. You also need to be level 10 or higher to enter Icelandis.
Icelandis is a forest with snow, frozen lakes, and many cold-themed mobs. Among the mobs is the famed Alpha Icewhal, known for dropping the Frostbite, a strong sword for it's level. The boss, Ra'thae the Ice King, drops the extremely expensive Frost-Forged Shield, and allows you to teleport to Taurarus.