All 25 Combos with Altar of the Brood

Felidar Guardian + Altar of the Brood + Spark Double

Infinite Mill: Infinite ETB, Infinite LTB, Infinite mill

Cast Spark Double by paying {3}{U}. Spark Double enters the battlefield as a copy of Felidar Guardian, triggering Altar of the Brood and itself. Resolve the Altar of the Brood trigger, causing each opponent to mill a card. Resolve the Spark Double's trigger, blinking Felidar Guardian. Felidar Guardian enters the battlefield, triggering Altar of the Brood and itself. Resolve the Altar of the Brood trigger, causing each opponent to mill a card. Resolve the Felidar Guardian's trigger, blinking Spark Double. Repeat from step 2.

Cards: Felidar Guardian, Altar of the Brood