It is the final boss of Hell.
While having an enormous amount of LP (5 billion), it certainly will pull the player to the limit with it spawning two Nature Mark IIIs (whom are the stronger brother of Nature Mark II), and dropping down 6-arrow-waves off-screen right above the closest character.
The Nature Mark IIIs that spawn from it are able to induce the "stone" ailment towards the characters by hurling spears at them and slowing down the characters by 5% for each spear. Fortunately, they only have 666 LP each, so it may be likely that they will be killed even before they shoot their first spear. Once the character is completely "petrified", the only way to cure it is by ingesting a Golden Onigiri or by dying/reviving during battle.
Besides that, the Behemoth can also summon one of two kinds of 6-arrow-wave dead-centered above the closest character. The grey one is the most lethal of all SR bosses, as it shoots down at a moderate rate (one per 2 seconds) and can quickly kill characters one by one. The red one is significantly weaker but creates a fire residue with high AT for 6 seconds. Melee characters are strongly advised that they will preform a dodge strategy and quick reflexes from the dropped arrows to avoid huge drops of health.
It would also be advisable to be above LV 100 to challenge the Behemoth.
Once defeated, it drops up to 3 Hell Chests and require a Hell Key to open it. These chests will provide a fire weapon that is exclusive from the standardized weapon list.
A massively powerful creature that once shared body with two others, in the form of The Beast. Hailing from Tenebrae, it experienced first-hand the chaotic non-chaos of its respective realm. When it emerged, Chronos divided the beast up into three lesser states, substituting them as the past, present and future. Being of the past, the first fragment which became known as Berengaria, was retroactively introduced into the universe as a sibling to Leviathan and Caledonia. The second fragment, which became known as Behemoth, was quickly captured by the archdemons. The third fragment, who became known as Apelpisia, was sent into the future to reappear at a later time.
During its captivity in Hell, Behemoth stood guard at the gate leading to the Tenebrae, only to leave his post once the beast fragment of the future arrives. This is why the archdemons are trying to keep Jacques Tenebrae out of Hell.