Reproduction Steps: 1. Deploy an unstoppable champion, e.g. Whirling Quarry. 2. Equip Slaver's Whip. 3. Deploy an enemy champion nearby. 4. Quarry will be unable to possess a champion as it is unstoppable and cannot become paralysed. @Sokolov @MogaBait can shed some more light about it. Solution: Add "This ability cannot be used if a possessing champion cannot be paralyzed."
You said it yourself. It's Unstoppable and therefore can't be Paralyzed. It's WAI. It's similar to Crystalized. Because it can't be Paralyzed, it prevents the entire effect.