Karn, the Great Creator, Doubling Cube, and Umbral Mantle

Prerequisites

• Karn, the Great Creator on the battlefield
• Doubling Cube on the battlefield
• Umbral Mantle on the battlefield
• Doubling Cube does not have summoning sickness
• available

Steps

1. Activate Karn's first loyalty ability by putting a loyalty counter on it, causing Doubling Cube to become an artifact creature until end of turn
2. Activate Umbral Mantle's equip ability by paying , attaching it to Doubling Cube
3. Float at least in your mana pool
4. Activate Doubling Cube by paying and tapping it, adding at least
5. Activate Doubling Cube by paying and untapping it, giving it +2/+2 until end of turn
6. Repeat from step 4

Results

• Infinitely large Doubling Cube until end of turn
• Infinite mana permanents you control can produce