Behemoth

The is a Life Spirit inhabiting a giant beast as its vessel, residing in the Demon Realm. It's said that when it's active, it swallows all life in his sight and possesses a portal to Camelot's dimension on his back, which awfully angers him.

Appearance
According to depictions, the Behemoth is a four-legged beast with a dragon-like head and two very long straight horns. On its back are mountain-like structures. It is depicted to be several times larger than the Demon King, who in his own right was gigantic.

Personality
The Behemoth is normally a peaceful being that lies asleep blessing demons with its life energy. However, when something upsets or scares it enough, it awakens and devours the life of everything around him.

History
Long ago, the Behemoth woke up from it's slumber due to the Demon King exacting his furious justice upon rebellious vassals, it started to cause destruction in the Demon Realm, consuming all life in it's path, but the Demon King eventually managed to force the beast back into it's deep sleep with his massive force.

After the Demon King was defeated by the Seven Deadly Sins in the new Holy War, a "sea change" was caused, which woke up the Behemoth again to spread disaster across the Demon Realm once more. During it's path of chaos, it's evil force was released across every inch of the land, felling the Demon Realm's plants, animals and even the Demons themselves.

Around the same time, a young boy named Percival arrived in the Demon Realm and was taken care of by the current Demon king and queen, Zeldris and Gelda, he was shown to be capable of giving Demons affected by the Beast's evil force, their life back, from that moment on, being seen as a saviour by the Demon Clan. Not too long after that, the Behemoth's energy reached Zeldris' castle, but it managed to push the beast's energy back for now. One day though, Percival disappeared from the castle and in some unknown way, managed to put the gigantic Behemoth back into it's deep sleep, saving the Demon Realm.

Demon Realm arc
When the Four Knights of Apocalypse and their allies go to the Demon Realm in search of an entrance to Camelot, Zeldris reveals that a portal recently opened on the Behemoth's back and this also caused the beast to awaken. After a week in which the rain of fire prevented the passage, the Knights go to the Behemoth, protected from its negative energy thanks to Percival. The latter is unconsciously able to make contact with the Behemoth's Life Spirits and by appeasing them with hope, the Behemoth falls asleep again.

Trivia

 * Behemoth is easily the largest creature in The Seven Deadly Sins universe to this day.