Gray Demons

The Gray Demon is a member of the Demon Clan, stated to be a higher being than the Red Demon. The corpse of the demon was used in Hendrickson'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 Hendrickson who studied the Demon's genealogy and biology and conducted countless experiments on its remains.

Kingdom Infiltration arc
After Ban sent Hendrickson 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 Gowther 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. Hendrickson 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 Hendrickson, the Gray Demon is a being higher than the Red Demon itself and its blood can provide the individual access to its strength, becoming a stable vessel of Demon power.