Abilities:
The Symbiote can bond with one crewmate, weakening their abilities and slowing their task completion, and they can choose to kill their bonded crewmate at any time. This ability is silent and leaves no body (like how scavenger's body gets teleported off the map).
Rules:
The Symbiote can only bond with one player at a time. Once bonded, they cannot bond with another until the bonded crewmate is killed or the Symbiote chooses to release them.
The victim remains unaware they are bonded until they start noticing slower task completion.
Slowing tasks completion examples:
Tasks resetting, Abilities not being gained after each task, tasks with a timer take longer, etc..
Balancing:
The Symbiote is limited to one or two bonds at a time depending on the host's settings, and of course Symbiote can be voted out.
Note: Task slowing can be done i think since it doesn't show anything about it in the "What is Impossible" section