Three Monsters

Three Monsters refers to the three most dangerous and powerful creatures in the Great Forest of Tob.

Background
While the three creatures held no alliances nor comradeship, the group known as Three Monsters derived its names from the denizens of the Great Forest of Tob, who saw them as the strongest monsters within the region. These three monsters resided in the forest, dividing it into territories which each one controlled.

It's unknown how long the Three Monsters had ruled over the Tob Forest. According to a certain adventurer, the first recorded existence of one of the trio began around two hundred years ago.

While the three shared the Great Forest of Tob as a home, alliances among them were nil and they constantly fought against each other over dominance of the forest. However, despite their power, they entered into a stalemate for an indeterminate length of time, unable to get an advantage over one another.

As Hamsuke's territory lies at the edge of the forest near the closest human settlement, E-Rantel, she was the sole creature of the Three Monsters to be documented by humans through tales and stories.

The Dark Warrior Arc
One of the Three Monsters made its debut when Momon traveled with a party of adventurers to the Great Forest of Tob to collect herbs. He heard about the tales of a powerful beast, the Wise King, that was said to possess magic and the ability to speak. Believing that such a beast would raise his status as an adventurer, Ainz set his eyes to capture it. Sadly the 'beast' ended up being just a giant hamster but Ainz decided to keep it due to it being a creature of reverence by the New World inhabitants. Christening it with the name of Hamsuke, the beast would later become Ainz's loyal pet and steed.

The Two Leaders Arc
It was later revealed that Hamsuke was just one of a triad of monsters that collectively ruled the forest. After her disappearance, the power balance in the Great Forest of Tob became severely altered. The two remaining leaders of the forest decided to put their centuries-old grudges aside and stand together. The Serpent of the West provided the tactical acumen, while the Giant of the East contributed the brute strength. Together they began to recruit the various creatures of the forest to their cause in a plan to attack the invader's stronghold: the Monument of Ruin.

However, their actions were noticed by Ainz Ooal Gown, who decided to personally settle the matter of the lords of the Great Forest of Tob. Ainz Ooal Gown attempted peaceful talks with the rulers of the forest but the negotiations ended with the death of the Giant of the East and the total submission of the Demon Snake of the West.

Strength
The Three Monsters are terrifying existences enough that they earned themselves the respect and fear by the Tob Forest inhabitants. They ruled the forest with fear and destroyed any that dared to enter. On average, all three of them were estimated to be around level 30.

Trivia

 * Most humans avoid venturing too deep into the forest due to the existence of these monsters.
 * So far, except for Hamsuke, the existence of the rest of the rulers were unheard of by humans.
 * The Northern Territory was ruled by no one, but Zy'tl Q'ae could have been considered as it's unofficial ruler.
 * Though Hamsuke was regarded as one of the Three Monsters, she had no prior knowledge of the group before her induction into Nazarick.
 * Almost everyone among the "Three Monsters," but Guu alone, is alive and have already pledged their loyalty to the Sorcerer King, Ainz Ooal Gown.