When a Shushu is bound to an object, it cannot escape until it is released. However, it can attempt to assume control of the body of any creature it comes in contact with. This is done easily with animals or even insects, but an intelligent creature can remain in control as long as its will is strong enough. However, any moment of weakness could lead to the demon taking over, which is why these objects are considered dangerous even for their owners and many are guarded by members of the Order of the Shushu Guardians.
A wielder may also voluntarily yield to the demon, but in most cases the latter will pursue its own agenda and never release its victim unless forcibly removed from the body. Sometimes this union has its own name, and a guardian who has tamed his Shushu may transform willingly and retain control of his actions.
Once a Shushu gains control of its host, the body undergoes a quick transformation into a mix of the creature's and the demon's appearances. This usually includes augmented strength or agility, a change in size, markings on the skin, etc. A person's voice usually changes to the demon's or something in between the two.
At the end of Wakfu season 2, Anathar takes control of Adamai's body, becoming a many-eyed Dragon who can hold his own against Phaeris.
In Wakfu season 2 episode 8, Justice Junior tells how Anathar has possessed his father. Though the Shushu is shown in his natural form, a short flashback sequence shows a transformation from the man to the demon with the intermediary stage shown in this image.
When Rubilax takes over Sadlygrove's body, their form is called Sadlyheart. Originally, Rubilax had near-complete control of his actions, but after he was tamed by his guardian, Sadlygrove retained full control in this form while benefitting from the Shushu's considerable strength.
In Wakfu season 1 episode 7, Rubilax takes control of Amalia's sickened doll while it's on a rampage.
In Wakfu season 1 episode 22, when he is abandoned and burrowed by Sadlygrove, Rubilax possesses a nearby caterpillar, turning it into a giant monster.
In Wakfu season 2 episode 21, while Sadlygrove has been turned into a primate by the Bellaphones, Evangelyne helps him take control of the Iop. Since Sadlygrove is already mindless in this state, Rubilax is able to fully control him.
In Wakfu season 1 episode 13, Skribble is left on deck of the ship and eaten by an Arachnee. By some process he chooses not to describe, he manages to take over the Arachnee and assume a humanoid form that can create spiderwebs.