Kentaro, the Smiling Cat + Cavern Harpy + Urza's Incubator + Rukarumel, Biologist

Kentaro, the Smiling Cat

Cavern Harpy

Urza's Incubator

Rukarumel, Biologist

How it works

1. Cast Cavern Harpy by paying 0.
2. When Cavern Harpy enters the battlefield, it triggers, returning itself from the battlefield to your hand.
3. Repeat.

This clever loop is built around Rukarumel, Biologist, who bestows the Sliver type onto all your creatures and creature spells everywhere, turning Cavern Harpy into a valid target for cost reduction. With Urza's Incubator naming Slivers, the generic cost of your creature spells is reduced by {2}, which neatly covers Cavern Harpy's mana value of 2. Meanwhile, Kentaro, the Smiling Cat bypasses the colored mana requirement entirely by letting you pay its mana value as {X}, allowing you to cast the Harpy for free as a generic {2}. When Cavern Harpy enters the battlefield, its own triggered ability forces you to bounce a blue or black creature—which can simply be the Harpy itself—allowing you to bounce and replay it indefinitely to generate infinite storm count and enters-the-battlefield triggers.