Rex, Cyber-Hound, Omen Hawker, and Horseshoe Crab

Prerequisites

• Rex, Cyber-Hound on the battlefield
• Omen Hawker in exile
• Horseshoe Crab in exile

Steps

1. Activate Rex using Omen Hawker's ability by tapping it, adding that can only be spent to activate abilities
2. Activate Rex using Horseshoe Crab's ability by paying , untapping it
3. Repeat

Results

• Infinite colorless mana that can only be spent to activate abilities