Boon of Fey Justice

You can turn other fey creatures into nymphs and cold riders (use the dryad and wight stat blocks). This functions in the same way as the true polymorph spell, except that its duration is instantaneous, shapechangers are not immune and the effects are automatically permanent. If the target has class levels, they retain them after the transformation.

Once you use this ability, you can't use it again until you finish a long rest.