Agatha's Soul Cauldron, Deeproot Pilgrimage, and Fatestitcher

Prerequisites

• Agatha's Soul Cauldron on the battlefield
• Deeproot Pilgrimage on the battlefield
• Fatestitcher in exile

Steps

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

Results

• Infinite ETB
• Infinite creature tokens