Aegis of the Legion

Mentor was built for creatures, not equipment: it lives on the attacking body, growing your smaller soldiers as your bigger ones swing. Grafting that keyword onto a piece of gear detaches the ability from any single creature and turns it into a repeatable line of play. Move the equipment each turn and any attacker can become the mentor, so a board that would normally have one counter-slinger now has a rotating one. The second ability is where the two colors earn their keep: every mentor trigger also drops a shield counter on the creature that received the +1/+1, which means the boosted attacker walks out of combat protected from the next instance of damage or the next effect that would destroy it. Boros aggro usually asks you to overcommit and hope; this asks the same overcommitment but hands back insurance on the creatures you invest in, softening the blowout that normally punishes a wide attack into open mana. Reassigning the engine is not free: at three to move, you pay a real premium each time you want a different attacker to carry it, so the reward for going wide is throttled by how often you can afford to redirect the gear.