The proposal Samiel offered to Xalithar and Zyra'kai to gain their freedom wasn't an Amazing Plan or without any risk.
According to him, they had to become his soul-bonded pets, or companions in formal. By agreeing to this plan, their freedom would be restricted, and would obey Samiel's commands excluding killing themselves. To be fair, they also couldn't do any harm to each other.
And in order to cancel their bond, both sides would have to agree to it. And Samiel promised to free them whenever they wanted.
However, there were other disadvantages for Xalithar and Zyra'kai in this as well. Their ranks would decrease to match Samiel's strength after becoming his soul-bound companions. Worse, they would have their rank lowered by a whole rank when they canceled the bond as a punishment. So, the risk was quite high for the two monsters. While Samiel didn't have to lose anything at all.
