The presumably artificially created by race of mysterious creatures hidden plane. It contains nearly infinite amounts of very flexible energy tightly bound to many Planes, mostly the Material Plane. To see how it is used to make magic read the article.
The Subplanes contain any possible in the Cluster kinds of energy.
Therme
This Plane contains thermal energy. Can give and consume it. So basically can be used for both fire and ice things.
Lux
The Plane contains protons. Can give and consume them.
Kinet
The Plane contains kinetic energy. Can give and consume it. Mostly would be used to move and stop objects.
Electra
This Plane contains electricity. Can be thought of as an infinitely powerful battery with infinite capacity. It has both negative and positive charge sides.
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Not all subplanes of the Arcane one are known, literally anything more could be hidden there...