Zaru

According to the Chronicles of the Zaru Wars, five hundred years prior the Empire had been invaded by the Zaru, a race of sadomasochistic squid-people that summoned tentacle monsters in battle.

One day a huge army of Zaru, a species which had never been seen in the world before, had emerged from the Dark Heart and without any warning and without any reason or attempt at parlay they had started methodically laying waste to everything they found on their way. The kingdoms of Medelan had been almost completely destroyed and  Boruvala Rodenheim was utterly destroyed, their population either slaughtered, forced to flee or captured and sacrificed to the Zaru’s dark gods, and the invaders had gained so much momentum that it seemed as if the Empire as a whole was in danger. However, the advance of the squidmen was halted at the city of Corania, and the passage Marcus was now reading actually detailed the events of the siege. Chapter 74

It was confirmed by ValetheHowl that Rodenheim was destroyed during that war (Discord, 21 May 2021).

Zaru Berserker
They were incredibly strong and dangerous creatures and they possessed all sorts of skills, from unparalleled regenerative capabilities to their ability to feed off the pain of other creatures (Chapter91).