Halcyon Grid is a Virtual World with a full 3D environment, where you may move about with a character (or avatar) in a 3D World accessed with a 3D Graphics Application on a Desktop/Laptop computer usually, commonly known as a "Viewer," to most people on other Halcyon/OpenSimulator Worlds, the most common of which is the
Firestorm Viewer for OpenSimulator.
The OpenSim version of which is compatible, is known as "FirestormOS", the 'OS' meaning "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 supports Halcyon Worlds, since Halcyon Grid was a fork from the Open Simulator Project in 2010.
Covenant Calendar Class
Everyone in the Covenant Calendar Class (previously "Covenant Calendar Club") is welcome to join. Have named one of the regions "Study the Calendar" and loaded an "OAR" from Linda Kellie of a Conference Center on the region.
Examples on other Worlds
Here are a couple more examples of the Firestorm Viewer before login with two other Halcyon Grids selected, the previous Amaryllis Grid (with a picture of a region that had an airship which I liked) & Gospel World (showing the peak of the church there). Which are in no way connected to this grid, and do not support this grid.
Amaryllis Grid & Gospel World Examples
Here are a couple in-world snapshots of the previous Amaryllis Grid & Gospel World, they were/are both the same type of Simulator known as 'Halcyon', only with different content.
Unfortunately there is a wait of an indeterminate time for a new plugin to be made for the new free Vivox 5 Voice service, courtesy of Unity which bought Vivox. So until that time, there is a Discord group/server created with a voice channel that can be used while a person is in-word. It's just like a conference call where everyone else connected can hear. There are a few other worlds still using the previous iteration of Vivox Voice, though unfortunately by the time I had requested an account for the previous free Vivox voice service, it was bought by Unity, and the folks at Unity stopped giving out accounts for the older Vivox service.