Umbral Mantle, and Marwyn, the Nurturer

Prerequisites

• Umbral Mantle on the battlefield
• Marwyn, the Nurturer on the battlefield
• undefined at most available

Steps

1. Activate Marwyn by tapping it, adding an amount of equal to its power
2. Activate Marwyn by paying and untapping it, giving it +2/+2 until end of turn
3. Repeat

Results

• Infinitely powerful creature until end of turn
• Infinite green mana