Keiga, the Tide Star, Nim Deathmantle, and Ashnod's Altar

Prerequisites

• Keiga, the Tide Star on the battlefield
• Nim Deathmantle on the battlefield
• Ashnod's Altar on the battlefield

Steps

1. Whenever an opponent controls a creature, activate Ashnod's Altar by sacrificing Keiga, adding
2. When Keiga dies, it and Nim Deathmantle trigger
3. Resolve the Keiga trigger, causing you to gain control of a creature an opponent controls
4. Activate Ashnod's Altar by sacrificing the creature you gained control of in step 3, adding
5. Resolve the Nim Deathmantle trigger from step 2, causing you to pay to return Keiga from your graveyard to the battlefield, and attach Nim Deathmantle to it
6. Repeat as desired and/or whenever a creature enters the battlefield under an opponent's control

Results

• Gain control of all creatures opponents control and sacrifice them
• Gain control of any creature that enters the battlefield under an opponent's control