Agatha's Soul Cauldron + Deeproot Pilgrimage + Fatestitcher

Agatha's Soul Cauldron

Deeproot Pilgrimage

Fatestitcher

How it works

1. Activate the nontoken Merfolk using Fatestitcher's ability by tapping it.
2. Deeproot Pilgrimage triggers, creating a 1/1 Merfolk creature token.
3. Resolve the Merfolk's ability, untapping the other creature.
4. Activate the other creature using Fatestitcher's ability by tapping it, untapping the nontoken Merfolk.
5. Repeat.