Kelsien, the Plague, Thornbite Staff, and Mer-Ek Nightblade

Prerequisites

• Kelsien, the Plague on the battlefield
• Thornbite Staff on the battlefield
• Mer-Ek Nightblade on the battlefield
• Thornbite Staff attached to Kelsien
• Kelsien has at least one +1/+1 counter on it

Steps

1. Activate Kelsien by tapping it, dealing 1 damage to target creature and destroying it due to Kelsien's deathtouch
2. The creature dies, triggering Kelsien twice
3. Resolve the first Kelsien trigger, causing you to get an experience counter
4. Resolve the second Kelsien trigger, untapping it
5. Repeat
6. You may also repeat this loop any time a creature enters the battlefield under an opponent control

Results

• Destroy any creature that enters the battlefield under an opponent's control
• Destroy any number of target creature(s) opponents control