Metamorphosis is a powerful spell utilizes by Demon Hunters and Warlocks. The spell involves invoking a massive amount of Fel Energy to temporarily transform the caster into a Demon themselves, vastly enhancing their abilities and magical strengths. The spell is also one of the most difficult spells on Azeroth to perform, as the Metamorphosis ability is just as capable of transforming its caster into a Demon permanently and losing their minds to Fel corruption.
The spell was arguably first developed by the Night Elf and first of the Demon Hunters, Illidan Stormrage. While he may have developed this ability, it was most commonly cast and implemented by his disciples, the Demon Hunters that would arise in the centuries to follow his imprisonment at the conclusion of the War of the Ancients. The Metamorphosis spell is unique amongst Fel Magic abilities as it seemingly creates no lasting corruption. The caster swells with a massive amount of Fel Magic, transform into a Demonic form, and upon their spells conclusion, the Fel Mgic abates, draining from their bodies and taking the Demonic corruption with it. How this is accomplished isn't entirely known.
Demon Hunters are known to bind Demons within themselves to act as a sort of internal battery for the Fel Magic, drawing upon this source to fuel their metamorphosis and releasing it from their forms when no longer needed. Metamorphosis is certainly not a spell without risks. The more Fel Magic that is funneled into the Metamorphosis spell, the closer the caster is driven to the edge of their soul's tolerance for corruption. Illidan Stormrage was known to have achieved a "perfect" Demon form which melded his Night Elven and Demonic forms perfectly, into a new shape that did not require him to change back to Night Elven form without the loss of his sanity or personality to Fel Corruption. The energy absorbed from the skull of Gul'dan is likely to be one of the only sources of Fel Magic strong enough to fuel such a transformation.