Agatha of the Vile Cauldron + Umbral Mantle + Earthcraft

Agatha of the Vile Cauldron

Umbral Mantle

Earthcraft

How it works

1. Activate a basic land by tapping it, adding 1.
2. Activate Earthcraft by tapping the creature with Umbral Mantle attached, untapping the basic land.
3. Activate the creature by paying 1 due to Agatha of the Vile Cauldron and untapping it, giving itself +2/+2 until end of turn.
4. Repeat.

This brilliant engine turns a simple untap loop into an infinitely large creature by chaining together mana generation and a self-contained pump ability. Earthcraft provides the crucial enabler, allowing you to tap your equipped creature to untap a basic land for mana, which then fuels the active sequence. Meanwhile, Agatha of the Vile Cauldron acts as the cost-reducer, shrinking the price of Umbral Mantle's untap-and-pump ability down to just one mana so you can perpetually repeat the cycle. As the creature keeps untapping itself via the Mantle, each iteration pumps its power higher and higher, leaving you with an unstoppable, arbitrarily massive threat ready to trample over any opposition.