Oni no Ou to Chigire - Chapter 2 Pg 62 & 63

Thats enough.

Yato told him. Tokimori took his mouth away from Yatos arm. The back of his throat and the pit of his stomach were hot. Badump, his heart beat loudly, and a heat like sparks ran through his whole body. His head hurt, he felt nauseous, but he desperately endured.

Demon masters take in the blood of a demon accepting their summons as a way to for them to connect to their demon servants. Contracted demons found in the Intersection of the Six Realms werent actually allowed to go to the human world, and could only be summoned for jobs. Once they were no longer being employed, they were sent back to the Intersection of the Six Realms. Once summoned, a demon servant will come out from the summoning plot into the human world, then relying on the presence of their own blood, he will search for his master and arrive in no time. Since Yato was a demon that was originally in the human world, Tokimori kept him by his side day and night. He didnt need to use the demon summoning plot, so it was possible to be bound by contract without the ceremony, but the Yase family considered it an illicit union and would not officially accept it. A demon must obey the demon master he bestowed his blood to as their personal servant unless their master dies, or dissolves the contract themselves. A contracted demon also was unable to take on another demon master.