Mega Aggron (5e Creature)
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Large elemental, any alignment
Siege Monster. The aggron deals double damage to objects and structures.
Heavy Metal. The aggron has advantage on saving throws to resist being moved.
Sturdy (Recharges after a Short or Long Rest). If the aggron takes 28 damage or less that would reduce it to 0 hit points, it is reduced to 1 hit point instead.
Filter. Whenever the aggron takes damage, it instead takes half as much damage.
Iron Tail. Melee Weapon Attack: +13 to hit, reach 5 ft., one target. Hit: 50 (12d6 + 8) bludgeoning damage. The target must succeed on a DC 21 Strength saving throw or be pushed back 10 feet.
Smack Down. Ranged Weapon Attack: +5 to hit, range 60/180 ft., one target. Hit: 21 (6d6) bludgeoning damage. The target must succeed on a DC 21 Strength saving throw or its flying speed (if any) is reduced to 0 feet for 1 minute.
Metal Burst. When the aggron takes damage from a melee attack, it can use its reaction to force the attacker to make a DC 21 Dexterity saving throw. On a failed save, the creature takes damage equal to the damage of the triggering attack, or half as much on a successful save.