How does one become sub cosmic?
The beings that could completely perceive, and manipulate ambient cosmic energy in their surroundings, were those known as 'Sub-Cosmic' beings.
One might wonder why they are called 'Sub'.
The reason is because of the existence of complete 'Cosmic Beings', however, that is a story for a later date.
Hades, Diablo, and Arthur.
One of the things the trio had in common was that they were all sub-cosmic beings.
Sub-Cosmic beings have the ability to physically see cosmic energy, hence the reason for Hades and Arthur's reaction when Diablo brought them into the space where they were currently seated.
They could see the cosmic energy that was being emanated from the core of the Hell universe.