Undra, Bastard of the Frosts

Trampling Charge. If the mammoth moves at least 20 feet straight toward 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 mammoth can make one stomp attack against it as a bonus action.

Flame Bringer. If the Mammoth misses two attacks with his Pick on a single target, the third will create a burst of Flame reaching 5ft outward from the pick. The target must make a DC 14 Dexterity save or take an additional 2d8 Fire damage on a failed save.

Combo. The Mammoth makes two Slashing attacks and one stomp attack. 

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 prone creature. Hit: 29 (4d10 + 7) bludgeoning damage.

Slash. The Mammoth slashes with its pick, +2 to hit 2d6 Slashing/Peircing Damage.

Kindling. The Mammoth takes an action to target itself with a Slashing attack, forcing itself prone for the round. However, for the following 8 rounds of combat all attacks gain the Flame Bringer trait nomatter the weapon or method of attack used. 

Roaring Trumpet. The Mammoth Trumpets out in an attempt to halt an attack, all players in a 50 foot range must make a DC 14 Constitution save or get disadvantage on all attack rolls for a round.

Couterattack. The Mammoth pulls back and takes a swing with its trunk, the target makes a DC 10 Strength or Dexterity check or take 2d10+1 Blugeoning Damage and be knocked back 10 feet.

Flames Favor (3/day) . The Mammoth chooses to succeed a failed save or attack roll.

Searing Halo. When severely weakened he calls upon the fire to aid him once more. Until he is restored to half of his maximum HP all creatures in a 50 foot radius will be required to make a DC 13 constitution save for every action in combat taken, on a failed save a creature takes 1d6+1 fire damage. While the Searing Halo is active, the Mammoth restores 1d4+2 of his HP per turn.