Corrupted Mammoth

Trampling Charge. If the corrupted mammoth moves at least 20 feet straight towards a creature and then hits it with a gore attack on the same turn, that target must succeed on a DC 18 Strength saving throw or be knocked prone. If the target is prone, the corrupted mammoth can make one stomp attack against it as a bonus action.

Lightning Absorption. Whenever the corrupted mammoth is subjected to lightning damage , it takes no damage and regains a number of hit points equal to the lightning damage dealt. 

Multiattack. The corrupted mammoth makes two stomp attacks. If both attacks hit a medium or smaller target, the target is grappled (escape DC 14), and the corrupted mammoth uses its Engulf on it.

Gore. Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: 25 (4d8 + 7) piercing damage.

Stomp. Melee Weapon Attack: +10 to hit, reach 5 ft., one target. Hit: 29 (4d10 + 7) bludgeoning damage.

Engulf. The corrupted mammoth engulfs a Medium or smaller creature grappled by it. The engulfed creature is blinded, restrained, and unable to breathe, and it must succeed on a DC 14 Constitution saving throw at the start of each of of the corrupted mammoth's turns or take 13 ( 2d8 +4) bludgeoning damage. If the mammoth moves, the engulfed target moves with it. The mammoth can only have one creature engulfed at a time.