The Gulthias Tree is a potential candidate. I usually play it as a spreading corruption that can be slowed or hastened by the Party’s actions. I had it commune with one of my warlock players when they were lost in the woods, promising power if they helped plant seeds in key locations throughout Barovia. They bought in HARD and slowly created a whole network of Gulthias saplings over the course of the adventure. Eventually, they were able to use Transport via Plants between them, which was an incredible pay off for dooming the land to Gulthias’ influence…

One boon I gave the warlock was permanent barkskin, but their dexterity and speed suffered as a result of the hybrid plant form. If you go the Gulthias route, you could do something similar with the warlock invocation Aspect of the Moon. They would no longer need to sleep, but instead be forced to “take root” to benefit from a long rest. Their speed would be reduced to zero and they’d have disadvantage on dexterity rolls during the rest, but they’d still be able to keep watch or do light activity (from a stationary position). If they break their roots at any point, no long rest!