Ancient Fallen Titan

Berserk. Whenever the titan starts its turn with 40 hit points or fewer, or after being called tiny roll a d8. On a 1, a 3 or a 8, the titan goes berserk. On each of its turns while berserk, the titan attacks the nearest creature it can see. If no creature is near enough to move to and attack, the titan attacks an object, with preference for an object smaller than itself. Once the titan goes berserk, it continues to do so until it is destroyed or regains all its hit points.

The titan's summoner, if within 60 feet of the berserk titan, can try to calm it by speaking firmly and persuasively. The titan must be able to hear its creator, who must take an action to make a DC 15 Charisma (Persuasion) check. If the check succeeds, the titan ceases being berserk. If it takes damage while still at 40 hit points or fewer, the titan does not go berserk again unless you call him tiny.

Aversion of Fire. If the titan takes fire damage, it has disadvantage on attack rolls and ability checks until the end of its next turn.

Immutable Form. The titan is immune to any spell or effect that would alter its form.

Lightning Absorption. Whenever the titan is subjected to lightning damage, it takes no damage and instead regains a number of hit points equal to the lightning damage dealt + 1D6.

Magic Resistance. The titan has advantage on saving throws against spells and other magical effects.

Magic Weapons. The titan's weapon attacks are magical.

Multiattack. The golem makes two smash attacks.

Smash. Melee Weapon Attack: +7 to hit, reach 10 ft., one target. Hit: 13 (2d8 + 4) bludgeoning damage.