The Hell Monsters are demons from the Hell dimension from the series written by MosuFan2004, Hell Expedition.
The dimension where humans are nowadays and the Hell dimension were once the same dimension, but that was a long time ago when humans didn't even exist. The dimension got separated in two: Hell dimension and the one where humans are now. The creatures of both dimensions were in the same dimension, and they were still evolving. When the dimensions got separated, the Hell Monsters evolved in a much different way, they gained demonic and scary powers and appearance. A portal between the Hell dimension and the other one appears once in some decades, and the only way to close it is to go to the Hell dimension and kill the Hell lord. Hell lord is a designation for the most powerful Hell Monster that opened the portal.