The Devil's Pledge is a "dominion" mark, so to speak, a symbol of power that only a high ranking demon would possess.

In terms of leadership magic, it's something of a proof of a demons’ submission to another; the mark and brand usually appears on the forehead.

This is, perhaps, one of the reason, demons have built a community based around who gets to reign supreme and carefully choosing who they marry, and it’s also one of the reasons why they are still dueling each to determine their positions.

However, in the end a pledge is only able to come into effect if the parties willingly agree to be bound by such a contract.

Crude Image of what it could look like

Tooru's Enslavement Magic, on the other hand, is completely different since, as long as the other party has an actually body, even if they did not pledge their submission to him he would be able to control them.

This is the true essence of what Palmyra was after when she wanted to use his Slavemancer abilities.

Also, for a demoness whom is connected to him by the pledge of loyalty, instead of eating a meal it is necessary for her to obtain her magical supply directly from Tooru, the Master, in order to sustain herself.