Gray Demons

The Gray Demon is part of the Demon Clan and is a higher being than the Red Demon. It's being used as one of Hendriksen's experiments.

Appearance
The Gray Demon is a tall being compared to a Giant's size, has long scaly arms, sharp claws and feet, and small wings. It wears a lower bottom armor and a sun-shaped helmet with a terrifying face.

History
At some point of time, the Gray Demon was killed by an unknown individual of great power as it is shown to have died from a large slash wound in the chest. It is shown to have participated in a war between its Clan and the Four Races.

A few years prior to the events of the Kingdom Infiltration arc, its corpse was discovered by Great Holy Knight Hendriksen who studied the Demon's genealogy and biology and conducted countless experiments on its remains.

Kingdom Infiltration arc
After Ban sent Hendriksen plummeting down deeper into the underground chambers beneath Merlin's ruined castle, the Great Holy Knight approached the Gray Demon's corpse explaining to the four Sins Meliodas, Ban, King and Gowthern how he obtained and experimented on the demon's body, only to conclude that the Gray Demon was a higher being when compared to its Red brethren. Hendriksen then proceeded to take a blood sample from the Gray Demon's corpse and inject it into himself, resulting in tremendous power-boost, prompting his counterattack by first taking out the Fox's Sin of Greed.

Abilities/Equipment
According to Hendriksen, the Gray Demon is a being higher than the Red Demon itself and its blood can provide the individual access to its powers of light and a resilient body to become a stable vessel of Demon power.