The Hundredth-Year Monster is a monster who appears every 100 years and attacks people. The Holy Aionian Empire was gathering troops to face this menace, but cities in the area had declared their independence because they were dissatisfied with the heavy burdens being imposed on them. However, urged by the coming advent of the Hundredth-Year Monster, The Holy Empire urged these cities to cooperate with eliminating the incoming threat.
When the Hero and his friends are summoned to the Holy Aionia Empire's capital and meet the Holy Emperor Leopherias XIII, they're informed that Aionians refer to the mysterious creatures as the Telluspharma (テラスファルマ Terasufarma?), a term used by Aionians for so long that they'd forgotten how the term was coined.
In actuality, Telluspharma came into existence about 300 years ago. Back then, there had a war between the Great Holy Empire who reigned over two continents and the Stars of Destiny, gifted individuals who received the blessings of power from The Original Tree of Times. The Empire noticed this unusual circumstance and researched a way to use the Tree for their own purposes. As a result of their research, Telluspharma was created from the Holy Empire's attempt on drawing the Original Tree of Times's power to rival the Stars of Destiny's strength. However, the resulting monsters cannot discern between friends and foes and prey on everything in their midst, even attacking their own kind.
As the Telluspharma begin to prey on everything, in a land faraway from the Empire's territory, Leopherias used the power of the Original Tree of Times and erected a barrier to protect everything that lives in that area. This barrier, although very strong, weakens every 100 years and allows the Telluspharma to infiltrate into the barrier. To anticipate that, the Holy Aionian Empire is formed and its soldiers are trained to face the monsters.
Telluspharma seemed to regard the Tree of Times as its natural enemy, so if the power of the Tree of Times weakens, the monsters grow stronger and vice versa. This is supported by the fact that at first, when the Hero and Myura escaped from Telluspharma into the Lake Fortress, they can't close in because of the power of the Tree of Times.