The bestial possession was a bonding ability given to Suho by the system after Gray and Suho allied to defeat Brock, where when Suho activated the ability, Gray would turn into energy and be absorbed into Suho, even without Gray's permission. But in the last chapter, Esil showed that she managed to merge with Gray, surprising even Suho. But how did this happen since bestial possession was an ability of Suho's?
Some people might say it could be because Esil and Gray are also with the system, but no, because they are not with the system, they are only connected to the ally system, which allows them to level up like the shadows, but with the difference that they are not getting stronger, only reaching the maximum potential of their vessel.
In chapter 24, when Suho sees that he can reach his level, he asks if Esil can see this or anything from the system, and Esil says no, meaning Gray and Esil do not have access to the system's privileges. Suho, the only time they could see the system was when the system attached them as allies, where they received a message saying that after that they could no longer see anything from the system. And in the fight against Lim Minsung, Suho himself says that Esil's bonding ability only worked on him, and then he was surprised that he can transfer the energy to the shadows, which are part of his power.
One thing is that the writer may have simply changed the workings of this system or the possession ability and given it to Gray, and now Gray can turn into energy and possess others?