Arboreal Tranformation (EFA - A)

You transform your target into a Huge oak tree. The creature’s skin splits open, revealing wood and bark. Leaves sprout from its limbs and it starts to grow. Choose a living creature within range, which must make a Constitution saving throw. On a failure, for a number of days equal to your spellcasting class level, the oak tree becomes a treant (though it can't animate trees) and obeys your commands. At the end of the duration, the subject becomes in all respects a normal tree, taking root where it stands if it can.

A creature making a successful saving throw does not transform. Instead, it is affected by the Slow spell  for the duration as its body fights off the transformation. At the end of the duration, the creature returns to normal.