Myrkul, Lord of Bones, and Devoted Druid

Prerequisites

• Myrkul, Lord of Bones on the battlefield
• Devoted Druid on the battlefield

Steps

1. Activate Devoted Druid's second ability by putting a -1/-1 counter on it, untapping it
2. Repeat step 1 until Devoted Druid dies as a state-based action due to having zero or less toughness
3. Myrkul triggers, causing you to exile Devoted Druid and create a token copy of Devoted Druid that is an enchantment and loses all other card types
4. Activate the Devoted Druid token's first ability by tapping it, adding
5. Activate the Devoted Druid token's second ability by putting a -1/-1 counter on it, untapping it
6. Repeat from step 4

Results

• Infinite green mana