To access the secret level simply interact with the library in the starting room in E5M4, which will unlock E5MX mission selection in the main menu. Of course you'll need to have found all of the required spell pages in previous levels. I put the missions together because they are basically the same thing, plus I actually found the secret level pretty disappointing.
0:00 - E5M4: Boss Fight Beelsebur II
0:29 - Ending
1:57 - E5MX: Boss Fight That Thing
Movement: WASD
Jump: Space
Dash: Shift
Primary Fire: Left Mouse Button
Change Weapon Type: Right Mouse Button
Crouch: Cltr
Bullet Time: Left Alt
Reload: R
Interact : Middle Mouse Button
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