Mutated Truffle

The Mutated Truffle is an Expert Mode-exclusive, post-Moon Lord accessory dropped by The Old Duke. It summons a miniature Old Duke minion to attack nearby enemies by ramming into them, dealing 2000 base summon damage. The mini Old Duke will enrage when the player is below 50% of their health, moving and ramming at a much higher speed, also indicated by the mini Old Duke's eyes turning yellow.

Trivia

 * The miniature Old Duke minion is referred to as "the young duke" in the buff's tooltip, and as such it lacks Old Duke's facial hair, presumably being too young to have grown it yet.