Amphibious. The dragon can breathe air and water.
Legendary Resistance (3/Day). If the dragon fails a saving throw, it can choose to succeed instead.
Shadow Hands (2/3 hp) dragon reduces all damage taken to 1. summons 4 massive shadow hands in four corners of the arena, each with 100hp. The hands have their own actions listed in legendary actions. Dragon will not take damage as normal until the hands are all defeated.
Skyward Assault (1/3 hp) dragon leaps up in to the air for 3 turns spraying down shadow breath on the arena. The dragon can occasionally land dealing 8 (2d8) to anyone within 20ft of the landing zone, and dealing 60 (10d12) if the dragon lands exactly on any character.
Cyclic Shadows (after skyward assault) .dragon calls in shadow flames towards its body, in a spiral 20ft radius. if the flames are touched they deal 67(15d8) necrotic damage. The flame wall is 20ft long as it spirals in.
Multiattack. The dragon can use its Frightful Presence. It then makes three attacks: one with its bite and two with its claws.
Bite. Melee Weapon Attack: +15 to hit, reach 15 ft., one target. Hit: 19 (2d10 + 8) piercing damage plus 9 (2d8) acid damage.
Claw. Melee Weapon Attack: +15 to hit, reach 10 ft., one target. Hit: 15 (2d6 + 8) slashing damage.
Tail. Melee Weapon Attack: +15 to hit, reach 20 ft., one target. Hit: 17 (2d8 + 8) bludgeoning damage.
Shadow Breath (Recharge 5–6). The dragon exhales acid in a 90-foot line that is 10 feet wide. Each creature in that line must make a DC 22 Dexterity saving throw, taking 67 (15d8) necrotic damage on a failed save, or half as much damage on a successful one.
Shadow Tendrils. Ranged attack, un-blockable. Shadow tendrils chase up to 6 targets and deal 18 (3d12) necrotic damage
The dragon 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 dragon regains spent legendary actions at the start of its turn.
Detect. The dragon makes a Wisdom (Perception) check.
Tail Attack. The dragon makes a tail attack.
Wing Attack (Costs 2 Actions). The dragon beats its wings. Each creature within 15 feet of the dragon must succeed on a DC 23 Dexterity saving throw or take 15 (2d6 + 8) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.
Shadow Hands. Melee line attack, hands grasp out of the shadows in a straight line, trying to grab and subdue anyone hit by them. dealing +13 to hit 25 (5d10) in a straight line 10ft wide. All hit must make a DC 17 STR saving throw or be grappled
Shadow Tendrils (aux) every time a hand is attacked by 3 individuals it lets out a shadow tendrils attack that hits the 3 nearest characters for 18(3d12) necrotic damage
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