With a Base Stat Total of over 1,000, Eternamax Eternatus is probably not meant to be owned by any trainer.
However, by hacking it into a raid, and setting the form to 1 (instead of the default 0), after it was caught in a raid battle,
it does not revert back to regular Eternatus.
(Even if the "send into box" and party/summary screen shows a regular Eternatus)
When you bring it out to battle, it mostly forces the game to try to render it within the skybox, with Eternamax Eternatus hovering far up.
It causes the camera to pan in some weird angles and sometimes you can see the below or into the other environmental objects.
The Eternamax Eternatus can be brought to the Isle of Armor and have it follow you, and a somewhat small version of it will follow you, when you set it to the lead in your party.
However, it will stay on the spot, and is unable to follow you. Thanks to Anubis (Sibuna_Switch) for testing this. (source video)
Important to note: loading any save with Eternatus in Eternamax mode will cause it to revert to standard form.
Having this in your save will make it illegal. As per any item in the Unreleased/Beta PKM Gallery, use at your own risk.