Xavier Sal, Infested Captain + Haunted One + Intruder Alarm + Ashnod's Altar

Xavier Sal, Infested Captain

Haunted One

Intruder Alarm

Ashnod's Altar

How it works

1. Activate Xavier Sal's first ability by tapping it and removing a +1/+1 counter from the additional nontoken Human, Fungus and/or Pirate if it has one, or any counter from another permanent you control otherwise.
2. Xavier Sal triggers, causing it and all other creatures you control that share a creature type with it to get +2/+0 and gain undying until end of turn.
3. Resolve the Xavier Sal ability, causing you to populate.
4. Activate Ashnod's Altar by sacrificing the additional Human, Fungus and/or Pirate, adding CC.
5. When the creature dies, its undying ability triggers, returning it from your graveyard to the battlefield with a +1/+1 counter on it.
6. When the creature enters the battlefield, Intruder Alarm triggers, untapping all creatures.
7. Repeat.

This brilliant loop hinges on the synergy between Xavier Sal, Infested Captain and Haunted One, which grants Xavier and its fellow tribe members undying whenever they become tapped. By tapping Xavier to pay for its own ability, you trigger the undying grant and remove a counter from your creature, setting it up to be sacrificed to Ashnod's Altar for colorless mana. When that creature dies and returns with a +1/+1 counter courtesy of undying, its entry triggers Intruder Alarm to untap everything—including Xavier Sal—readying the entire engine to repeat. Meanwhile, Xavier's ability populates your board, allowing you to steadily multiply your token threats while generating infinite mana, sacrifices, and enters-the-battlefield triggers.