Treants newly sprouted. Some naturally, some in ways more grim. The Volys are usually numerous with short lifespans from when they first sprout. Like all plants, they find growth and long life in fertilizer made from a nutritious carcass.
The Volys are numerous creatures of the Fey that hold no true allegiance save for the protection of their mother and father trees. Though they are easily influenceable and growable. Torgothrin's plan to eradicate Men from the vicinity of his forest thus comes with the Volys, having instilled their seeds into the food and water supplies of the townships.
When a seed enters the body of a human, it repels the stomach acids that would dissolve most things. It sits there instead, beginning to consume the food intended for the human like a parasite. This drives its host only to be more ravenous, in turn growing the seed quicker. The hunger and the growing seed mean illness quickly befalls the host. But before they truly perish the seed takes control of their mind, taking them to a tree or a free patch of earth where the seed suddenly sprouts, encasing the host in wood and consuming them in a seemingly cocoon-like state. When the cocoon is opened again, a Volys stands in place of the man that it consumed.
No known magic or restorative medicine can remove the seedlings, only surgery where the seed itself is cut from the stomach of the host. A procedure that few can survive.

Volys - Trio of Treant Saplings [25mm bases] is created by DM Stash and suitable for any table top rpg or wargame.

Looking for some models in the same set check Feywalkers  collection.