Halcyon World is a Virtual World with a full 3D environment, where you may move about with a character(avatar) in a 3D World accessed with a 3D Graphics Application, commonly known as a "Viewer," to most users of these Worlds, the most common of which is the Firestorm Viewer.

The OpenSim version of which is compatible, which is known as "FirestormOS" the 'OS' standing for "OpenSim" in this case, not to be confused with "Operating System" in other contexts. While this World is not technically an "OpenSim" Grid with the ability to teleport between Grids, the FirestormOS Viewer also works for Halcyon Worlds, since Halcyon World was a fork from the OpenSimulator Project in 2010.

For a couple more examples, here are two Screenshots of the Firestorm Viewer before login with two other Halcyon Grids selected, Amaryllis Grid & Gospel World. Which are in no way connected to this grid, and do not support this grid. Unfortunately the owner of Amaryllis Grid shut down the world, without any notice nor information at all. So a group of the residents have started Sovaria Estates. It's ran by a board with a non-profit organization setup for it.

Here are a couple in-world snapshots of the previous Amaryllis Grid & Gospel World, they are/were both the same Halcyon type of World Simulator, just with different content.