Abyssal

Abyssals are demonic constructs similar to infernals, but made from rougher, more jagged material. Their flames can be either red, green or even blue.

The ritual to summon an abyssal is complicated and often fails. When a failure occurs, a small piece of the demon typically remains behind, seething with a combination of latent demonic rage and despair at its own dysfunction. Ancient battlegrounds are littered with such stones.

Their white-hot cores can be useful as a compressed fuel source.

As a companion pet

 * Netherspace Portal Stone has a chance to drop from Prince Malchezaar in Karazhan.
 * Shard of Supremus has a chance to drop from Supremus in Black Temple.

Speculation

 * The differences in the color of the flames of infernals, abyssals and Supremus could be tied to what sort of elemental was bound within. Perhaps abyssals are created from more powerful elementals and Supremus from an even more powerful elemental.
 * Alternatively, it could depend on the magic used to create or power them. For example, Illidan fueled his own Demonic legions with the arcane power of the Shrine of Lost Souls, which may have lead to the blue color of Supremus' flames.