Ceratopper
This adamantine helm is fashioned in the likeness of a triceratops and decorated with painted designs. You have a +1 bonus to AC while you wear this helmet. If you move at least 20 feet toward a target and then attack it with a melee weapon attack on the same turn, you can choose to make a special attack using the helmet instead of the weapon. On a hit, the target takes piercing damage equal to 3d4 + your Strength modifier. You’re proficient with this attack and use your Strength modifier for its attack rolls. If you hit and the target is no more than one size larger than you, it must also succeed on a DC 13 Strength saving throw or be knocked prone.
In addition, while wearing the helmet, you can use an action to cast the polymorph spell from it, transforming yourself into a triceratops. Once this property has been used, it can’t be used again until 2 days have passed.
He donned the helm and stared down his muscled foe. One step. Two steps. Three. Four. He was running now, fully bounding between paces. The adrenaline was coursing through his system and he could feel the thrill of the incoming impact. A crunch, and then laughter. The hulking orc still stood before him, towering and imposing as ever. The helm pierced his armor, but the orc's resolve stood stronger than his. "IS THAT BEST MAN CAN DO? PUNY." The orc chortled with a sneer. He made no move the retaliate. The man pulled back and replied, his voice shaking with his panting breath, "Perhaps...it is. Maybe this...can do...better." His form began to twist and reform itself. His skin bubbled and hardened into a scaled armor. What was once a puny man was now an indominable monster of prehistoric proportions: a triceratops. With an earth-shaking roar, the newly formed titan bore down again on the orc: his smirk now a look of deep horror and regret.