Ilshenar
Ilshenar is a facet, or landmass, of the UO map. Introduced with Third Dawn, Ilshenar was originally only able to be visited with the 3D client. With the Blackthorn's Revenge expansion the 2D client was changed to allow access to Ilshenar.
Ilshenar remains unique in the fact that it is the only facet you can not Recall, Gate Travel, or Sacred Journey into. Gate Travel cannot be used to leave Ilshenar, but the other travel spells can. Other than this, Ilshenar mostly follows the Trammel rules set.
Ilshenar has been described as "one big dungeon," because with few exceptions it exists to hunt monsters.
While there are Champion Spawn locations in Ilshenar, Power Scrolls do not drop. There is little reward for doing them there other than Champion Titles (e.g., John Doe, Foe of the Abyss or Jane Doe, Eradicator of the Vermin Horde).
Paragon creatures spawn in Ilshenar, along with the minor artifacts that are rewards for killing paragons.
Fiction
The fiction/lore of Ilshenar has changed since the map was introduced.
For the original lore, see BNN: An Introduction to Ilshenar - Part I. This lore revolved heavily around the abilities of Ancient Ilshenarians to travel easily between facets and even potentially between shards. However, this lore was erased from the UO.com Internet site, with the fictional explanation being that Blackthorn and Exodus had altered the timeline, by bringing the Juka race into our present.
Cities, Towns, and Villages
Ilshenar is home to the gargoyle city Vel Lor Reg, which translates as "Home of Light." The Ethereals reside in Reg Volom.
There is a city of pixies near the Spirituality Shrine, and several ruined cities within which monsters spawn. Lakeshire is one of two homes to the good-aligned Meer race. Within Montor, various demonic and fire-based creatures spawn. In Mistas, the Juka and other creatures affiliated with Blackthorn, such as Chaos Dragoons and Chaos Dragoon Elites, have taken up residence.