Infernal energy is the essence of the Hells and other evil planes, other than the Void or the Rending. There are many ways to access it, but the most effective is through the use of others.

The Hells provide pure infernal energy, infernum
The Catacombs provide punishing energy, called arnum
Grey Lands provide undeath energy (Msawhat - the corrupting essence of the Grey Lands)
Evernight provides shadow energy
Broken Obelisk provides pain energy, also called arnum
Void - from The Void
The Rending - from the Rending
The Abyss provides entropy
The Cenotaph provides msawhat

Though the Hollow Realm is infernal, literally nothing is there.