Argynvost, The Fallen Beacon

The tortured remnant of Argynvost, corrupted by centuries of imprisonment, hatred, and the desecration of his remains. What remains of the great silver dragon recognizes neither friend nor foe. Only by destroying this corrupted manifestation and returning Argynvost's stolen skull to its rightful resting place can his true spirit finally be freed.

Multiattack. Argynvost uses Frightful Presence. He then makes three attacks: one with his Bite and two with his Claws. He can replace one Claw attack with a Tail attack.

Bite. Melee Weapon Attack: +17 to hit, reach 15 ft., one target. Hit: 21 (2d10 + 10) piercing damage plus 14 (4d6) necrotic damage.

Claw. Melee Weapon Attack: +17 to hit, reach 10 ft., one target. Hit: 17 (2d6 + 10) slashing damage.

Tail. Melee Weapon Attack: +17 to hit, reach 20 ft., one target. Hit: 19 (2d8 + 10) bludgeoning damage.

Frightful Presence. Each creature of Argynvost's choice within 120 feet of him and aware of him must succeed on a DC 21 Wisdom saving throw or become frightened for 1 minute. A frightened 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 Argynvost's Frightful Presence for the next 24 hours.

Deathly Frost Breath (Recharge 5–6). Argynvost exhales corrupted silver-dragon frost in a 90-foot cone. Each creature in that area must make a DC 24 Constitution saving throw, taking 63 (14d8) cold damage plus 27 (6d8) necrotic damage on a failed save, or half as much damage on a successful one.

Breath of the Fallen (1/Day). Argynvost exhales the tortured spirits of his fallen knights in a 60-foot cone. Six spectral knights appear in unoccupied spaces of Argynvost's choice within the cone. Use the Phantom Warrior stat block for these creatures. The spectral knights act immediately after Argynvost in initiative and remain until destroyed, until Argynvost is reduced to 0 hit points, or until Argynvost dismisses them.

Gravebound Flight. Argynvost moves up to half his flying speed without provoking opportunity attacks. During this movement, he can move through the spaces of other creatures, but can't end his movement there.

Spectral Rebuke. When Argynvost takes damage from a creature he can see within 60 feet of him, he can force that creature to make a DC 21 Wisdom saving throw. On a failed save, the creature takes 18 (4d8) psychic damage and can't take reactions until the end of its next turn. On a successful save, the creature takes half as much damage.

Argynvost 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. Argynvost regains spent Legendary Actions at the start of his turn.

Detect. Argynvost makes a Wisdom (Perception) check.

Tail Attack. Argynvost makes one Tail attack.

Spectral Charge (Costs 2 Actions). The spectral image of a fallen Knight of the Silver Dragon appears at a point Argynvost can see within 60 feet and charges in a 30-foot-long, 10-foot-wide line. Each creature in the line must make a DC 21 Dexterity saving throw, taking 22 (4d10) force damage and falling prone on a failed save, or taking half as much damage without falling prone on a successful one.

Wing Attack (Costs 2 Actions). Argynvost beats his ruined spectral wings. Each creature within 15 feet of him must succeed on a DC 25 Dexterity saving throw or take 17 (2d6 + 10) bludgeoning damage and fall prone. Argynvost can then fly up to half his flying speed.

The Beacon Extinguished. When Argynvost is reduced to 0 hit points for the first time, he does not die or fall unconscious. Instead, his skeletal body collapses as brilliant silver light erupts from his remains. For a fleeting moment, the corruption disappears and the magnificent spectral form of the silver dragon Argynvost rises above the battlefield. Then shadow pours through the light, his bones rise again, and his anguished spirit roars. Argynvost regains 250 hit points, ends all conditions and effects currently affecting him, and immediately recharges Deathly Frost Breath. For the remainder of the encounter, Argynvost can use the following Mythic Actions.

Cry of Argynvost. Argynvost releases a mournful draconic roar. One creature he can see within 60 feet must succeed on a DC 21 Wisdom saving throw or become frightened of Argynvost until the end of its next turn.

Shattered Beacon (Costs 2 Actions). Silver-white light erupts from a point Argynvost can see within 90 feet. Each creature within 15 feet of that point must make a DC 22 Dexterity saving throw, taking 27 (6d8) radiant damage plus 13 (3d8) necrotic damage on a failed save, or half as much damage on a successful one.

Wings of the Silver Dragon (Costs 3 Actions). Argynvost becomes a streak of spectral silver light and flies up to his flying speed without provoking opportunity attacks. Each creature whose space Argynvost passes within 5 feet of during this movement must make a DC 22 Dexterity saving throw, taking 22 (4d10) cold damage on a failed save, or half as much damage on a successful one. A creature can take this damage only once each time Argynvost uses this action.