Tidespout Tyrant + Mox Opal

Tidespout Tyrant

Mox Opal

How it works

1. Cast Mox Opal by paying its mana cost.
2. Tidespout Tyrant triggers, returning a nontoken permanent that can be cast using one colored mana from the battlefield to your hand.
3. Activate Mox Opal by tapping it, adding one mana of any color.
4. Cast the nontoken permanent returned in step 2 by paying its mana cost.
5. Tidespout Tyrant triggers, returning Mox Opal from the battlefield to your hand.
6. Repeat.