Fel corruption

Prolongued use of fel magic may corrupt the users. It seems that fel magic has a tendency of turning users and those that spend enough time in the presence of those users into a more uniform demonic form: large claws, red tented skin, horns, fangs, ect.

Examples of this transformation are the orcs, blood elves, eredar, and to an extent, satyrs. It is also notable that out of the many Burning Legion races only a few have been seen uncorrupted.

Fel-Corrupted
Satyrs are night elves who have been corrupted by demonic powers either as a reward or as punishment by Sargeras, other races may also have been transformed later.
 * Night Elf - Satyr

Felblood elves are elves changed by siphoning fel energy from Suspended Terrorguards, they acquire new skin colours and horns and in some cases wings.
 * Blood elf - Felblood Elf

Fel orcs are orcs who have been changed by the Blood of Mannoroth and of Magtheridon, changing they're skin and various other mutations and making them into a blood-thirsty race.
 * Orc - Fel Orc

A ancient corrupted by the burning legion.
 * Ancient - Corrupted Ancient

The uppermost rank of the Burning Legion. Some, like (Kil'jaeden and Archimonde), were turned into demons by the dark titan Sargeras.
 * Eredar - Man'ari Eredar

Helboars are boars corrupted by the resident demons in Outland.
 * Boar - Helboar

These vile creatures are canines having somehow been twisted by demonic energy.
 * Unknown - Darkhound