Aspect of Tristram

Berserk. Whenever Tristram starts its turn with 60 hit points or fewer, roll a d6. On a 6, Tristram goes berserk. On each of its turns while berserk, Tristram attacks the nearest creature it can see. If no creature is near enough to move to and attack, Tristram attacks an object, with a preference for an object smaller than itself. Once Tristram goes berserk, it continues to do so until it is destroyed or regains all its hit points.

Immovable Force. Tristram is immune to any spell or effect that would force him to move unwillingly.

Blasted Casters. Tristram has advantage on saving throws against spells and other magical effects.

Multiattack. Tristram makes two slam attacks.

Vestige of The Schism. Tristram also wields The Titanstone Knuckles in their awakened state. 

Slam. Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: 16 (2d10 + 6) bludgeoning damage. If the target is a creature, it must succeed on a DC 15 Constitution saving throw or have its hit point maximum reduced by an amount equal to the damage taken. The target dies if this attack reduces its hit point maximum to 0. The reduction lasts until removed by the greater restoration spell or other magic.

Echos of Rage (Recharge 5–6). Until the end of its next turn, Tristram magically gains a +2 bonus to its AC, has advantage on Dexterity saving throws, and can use its slam attack as a bonus action.

Blueberry Pie (3/Day). Tristram regains 20 hit points, and he gains resistance to bludgeoning, piercing, and slashing damage until the end of his next turn.

I'd Like To See You Try. As a reaction to being the sole target of an attack (not subjected to an area of effect), Tristram can use its reaction to dash up to half of its movement without provoking an opportunity attack and make one Slam attack against a creature of its choice. 

Well Hello, Little One. Tristram leaps into the air and lands with a thunderous blast. All creatures within a 10 ft radius of where Tristram lands must make a DC 14 Dexterity saving throw or take 10 bludgeoning damage and fall prone on a failure. On a success, they take half the damage and do not fall prone. 

The Darkness Cannot Abide. If Tristram fails a saving throw, he can expend a use of his Blueberry Pie without gaining its benefits (no action required) and succeed instead.