Combined

Berserk. Whenever the combined starts its turn with 70 hit points or fewer, roll a d6. On a 6, the combined goes berserk. On each of its turns while berserk, the combined attacks the nearest creature it can see. If no creature is near enough to move to and attack, the combined attacks an object, with preference for an object smaller than itself. Once the combined goes berserk, it continues to do so until it is destroyed or regains all its hit points.

The combined's creator, if within 60 feet of the berserk combined, can try to calm it by speaking firmly and persuasively. The combined 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 combined ceases being berserk. If it takes damage while still at 40 hit points or fewer, the combined might go berserk again.

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

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

Legendary Resistance (3/Day). If the combined fails a saving throw, it can choose to succeed instead.

Multiattack. The combined makes two warhammer attacks and two great-sword attacks.

Warhammer. Melee Weapon Attack: +10 to hit, reach 5 ft., one target. Hit: 19 (2d10 + 8) bludgeoning damage.

Greatsword. Melee Weapon Attack: +10 to hit, reach 5ft., one target. Hit:  19 (3d6 + 8) slashing damage.

Charge. If the combined moves at least 20 feet straight toward a creature and then hits it with an attack on the same turn, that target must succeed on a DC 19 Strength saving throw or be knocked prone. If the target is prone, the combined can make one attack against it as a bonus action.

Frightful Presence. Each creature of the combined's choice that is within 120 feet of the dragon and aware of it must succeed on a DC 19 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature's saving throw is successful or the effect ends for it, the creature is immune to the combined's Frightful Presence for the next 24 hours.

The combined can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature's turn. The combined regains spent legendary actions at the start of its turn.

Warhammer Attack: The combined makes a warhamer attack.

Greatsword Attack: The combined makes a greatsword attack.

Frightful Presence: The combined can use its Frightful Presence ability.