Steelswarm Operator + Agatha's Soul Cauldron + Morphling

Steelswarm Operator

Agatha's Soul Cauldron

Morphling

How it works

1. Activate Steelswarm Operator's second ability by tapping it, adding that can only be spent to activate abilities of artifacts.
2. Activate Steelswarm Operator using Morphling's ability by paying , untapping it.
3. Repeat.

This clever engine comes together by feeding Morphling's activated untap ability into Agatha's Soul Cauldron, granting those self-same untap capabilities to any creature with a +1/+1 counter on it—including Steelswarm Operator. Once Morphling's card is exiled inside Agatha's Soul Cauldron to supply that text, Steelswarm Operator can tap for two blue mana and immediately untap itself by spending one of them, generating an infinite loop of restricted mana. Because Agatha's Soul Cauldron also allows you to spend your mana as though it were any color to activate creature abilities, it neatly bridges the color requirement to endlessly fuel the loop.