Type: Custom sky camera
Connections: None (Inherits from a built-in (G)ZDoom actor.)
Spawn Offsets: Where the camera will be is determined by the mapper themselves.
Spawn Ceiling: No, can be raised to it in-editor.
Variants: None
Summon: ParallaxingSkyViewpoint
Brightmaps: N/A
Actor Modification: Only inherits from the pre-existing SkyViewpoint that is inside the engine archive.
ACS: No

Code: inkoalawetrust (Rewritten code), DrPyspy (Original code)
Sprite Edit: Custom sky camera graphic by Tabijaky, made from GZDB/UDB's sky viewpoint graphic.
Idea Base: The Source engines' 3D skyboxes.

This actor is an improved version of the built-in ZDoom sky viewpoint: https://zdoom.org/wiki/Classes:SkyViewpoint. If you already know how to use the built-in sky camera, you'll be able to use this one with little issue, as long as you also read these instructions on how to use it below.
The parallaxing skybox camera only has one extra custom argument, located at the Action/Tag/Misc tab on the Edit Thing menu.
This argument is Sky Parallax, which controls how much parallax the sky camera has, AKA how much the skybox will move around when the player moves, the higher the value, the more the camera moves relative to the player, the lower the value, the less it moves.
For example, a value of 128 or 256 may be ideal if your skybox has things like buildings and hills, that are supposed to be far away from the playable portion of the map.
If you have issues like the player moving forward but the sky camera moving sideways, change the sky cameras' angle to 0.
You can also use the included skyparallaxexample.wad map as a reference on how to use the sky camera.
The default value for the Sky Parallax argument is 64.