Masks and Defense Mechanism
There was one artefact the betrayer had said they found in the dungeon that sent the kingdom of Edinburgh into an uproar. That was because it was something that could jeopardize the border security of Edinburgh along with other forces who shared borders with Badlands.
A monster-controlling artefact that could help its user enslave monsters of the same rank as them. Although it was supposedly limited to D-Rank, the artefact used a different method to enslave monsters than soul-binding mana contracts.
A typical soul binding contract wouldn't work on monsters anyway. In addition, if the ranker already had a set number of slaves, they couldn't add more without burdening their souls more. After a certain limit is crossed, the soul of the ranker who signed slave contracts with an excessive number of slaves than he could handle would break. This would turn them into lunatics.