Freid explored the Great Labyrinth in the Frost Caverns of Schnee, and managed to acquire Ancient Magic which allowed him to control Monsters, but at the cost of losing many of his subordinates to the Labyrinth's trials. He then began to use this newfound power to tip the scales of Demons' war against humanity in the Demons' favor.

When the heroes arrived at Tortus in order to fight the demons, he dispatched his subordinate Cattleya with a detachment of Monsters to the Great Orcus Labyrinth where they were training in order to either recruit or kill them, as well as to obtain the Ancient Magic in contained in the process.

(Source: Arifureta Wiki)