Hakumen no Mono is the main antagonist of the manga, Ushio and Tora.
The Hakumen no Mono was born of the hatred of humanity in ages past, and grew jealous of the warm chi that humans produced. To this end, it went to destroy countless civilizations throughout history. No matter what humanity, and even fellow youkai tried, nothing could stop him. Not until the Beast Spear was forged, its creator practically merging himself with it to create a weapon capable of defeating the dreaded demon. Injured by the Beast Spear and its wielder, it fled.
500 years before the series started, pursued by the latest wielder of the Beast Spear, it tried to attack Japan. Japan's youkai rose in defense of their homeland by the thousands alongside the forces of humans. However, just before Hakumen could reach Japan the Beast Spear struck and he was forced into the sea. Injured he went to spite the youkai and humans who rose against him...and bound himself to the Japanese Isles, planning to, upon healing dislodge himself...letting Japan sink beneath the waves. However, as he was healing a skilled priestess erected a powerful barrier around him and kept him imprisoned. For five hundred years, this cycle continued until the main protagonist's, Ushio's, mother had been chosen as the next barrier maiden. She maintained the barrier until her son was a teenager and had claimed the Beast Spear as his own...and tentatively befriended the Bakemono Youkai known as Tora (Nagatotobimaru to the Youkai).
Over this time, Hakumen's tails had split from his body and became sentient, evil demonic entities which would rage over and around Japan as his true awakening approached.
Hakumen possesses immense strength, speed, and durability, even compared to most powerful youkai. Even Azafuze, demons born to fight and defeat him from wielding the Beast Spear, are unable to cause lasting damage on their own, with only Tora being able to fight the beast. Even then, he was brought to near death. Hakumen's might was so great, the combined forces of the Youkai of Japan, the Kouhmei Buddhist Sect, and the anti-Youkai weapon equipped JSDF could only slow him down. Indeed, the only weapon that was capable of doing lasting damage to him was the Beast Spear itself, though at one point he had became immune to its effect (though this was temporary).
He also possessed the greatest firepower of any youkai in the series, able to wipe out entire islands with a single blast of his breath weapon, and doing it twice over to cause more hatred and fear of him. Due to being an entity of fear and hatred, the more that existed towards him, the stronger he became. The only way he was truly defeated was when the despair and hatred of humanity and the youkai was snuffed out by the wave of hope for Ushio and Tora to defeat him, countering his ability to regenerate from the Beast Spear's attacks.
As mentioned above, he is able to produce microscopic youkai which he used to wipe the mind of nearly everyone who met Ushio and Tora which blocked their memories of him. And his tails could split off from his body to form sentient entities which furthered his will, notably the Ayakashi (a serpent several miles long which ate entire ships).