The carrionette listed in Van Richten's Guide To Ravenloft, is given a Challenge Rating 1 (!?!) and features a Soul Swap ability that seems very powerful:
Targeting a creature within 15 feet that's been cursed by its Silver Needle (which has +4 to Hit, DC 12 Cha save, 1 min duration with no recurring saves), the carrionette can use its action to perform Soul Swap on that target:
The carrionette targets a creature it can see within 15 feet of it that is cursed by its Silver Needle. Unless the target is protected by a protection from evil and good spell, it must succeed on a DC 12 Charisma saving throw or have its consciousness swapped with the carrionette. The carrionette gains control of the target's body, and the target is unconscious for 1 hour, after which it gains control of the carrionette's body. While controlling the target's body, the carrionette retains its Intelligence, Wisdom, and Charisma scores. It otherwise uses the controlled body's statistics, but doesn't gain access to the target's knowledge, class features, or proficiencies.
If the carrionette's body is destroyed, both the carrionette and the target die. A protection from evil and good spell cast on the controlled body drives the carrionette out and returns the consciousness of both creatures to their original bodies. The swap is also undone if the controlled body takes damage from the carrionette's Silver Needle.
Neither the Silver Needle nor the Soul Swap descriptions mention limited uses, nor any limit to how many creatures can be affected.
It looks like the carrionette could have up to 10 creatures cursed with its Silver Needle? (1 use per round, 10 rounds per minute, 1 minute duration.)
Furthermore, I don't see any phrasing that specifically prevents the carrionette from using Soul Swap on another cursed target, while it is already controlling another target's body…
So, presuming everyone's within 15 feet of each other:
If a carrionette is already in control of a creature's body, and the carrionette uses Soul Swap on another cursed target… who ends up in which body?
Best Answer
The carionette cannot use Soul Swap when it is posessing a victim's body.
The Soul Swap ability states
The Soul swap ability as an action in her stat block is part of the carrionette's game statistics, not of those of the victim. Other than its mental abilitiy scores, when it is in possession of a body, it has to use the controlled body's statistics. These statistics do not have a Soul Swap action.