Amphibious. The young adult sea dragon can breathe air and water.

Siege Monster. The young adult sea dragon deals double damage to objects and structures.

Spellcasting. The young adult sea dragon is a 8-level spellcaster. Its spellcasting ability is 13 (spell save DC 15, +7 to hit with spell attacks). The young adult sea dragon has following spells prepared:
Cantrips (at will): Invisibility, Frostwave
1st-level spells (5 slots): Fogbreath (BA)

Invisibility. The young adult sea dragon can cast Invisibility as a cantrip, with concentration for up to 1 hour. A creature or monster you touch becomes invisible until the spell ends. Anything the creature or monster is wearing or carrying also turns invisible as long as it is on the target's person. The spell ends after 1 hour, or if the target attacks or casts a spell.

Frostwave. An icy torrent of water deluges in a 15 ft. line that is 5 ft. wide. Each creature within the affected area must make a DC 15 constitution saving throw. With a failed roll, the target takes 2d8 cold damage, and the target takes half that with a successful roll.

Fogbreath. A cloud of rolling fog emits from the young adult sea dragon's mouth. Range: 120/20 ft., targets in range must succeed on a DC 15 constitution saving throw or have disadvantage on attack rolls and skill checks for the duration. Duration is for one minute or until concentration drops.

Spellcasting.

Multi-attack. The young adult sea dragon makes two attacks, a combination of a claw, bite, or tail attack.

Bite. Melee Weapon Attack: +7 to hit, reach 10 ft., 1 target. Hit: 12 (2d10 + 2) piercing damage.

Claw. Melee Weapon Attack: +6 to hit, reach 5 ft., 1 target. Hit: 8 (2d6 + 2) bludgeoning damage. 

Tail. Melee Weapon Attack: +6 to hit, reach 15 ft., 1 target. Hit: 11 (3d6 + 2) bludgeoning damage. If the target is a creature, it must succeed on a DC 15 strength saving throw or be pushed up to 20 ft. away and knocked prone.

The young adult sea dragon can use any melee attack once (no Multi-Attack) as a bonus action on it's turn. The monster is also able to cast Fogbreath: cantrip (at will).

Multi-Attack of Opportunity. The young adult sea dragon can use it's reaction to perform a multi-attack action as an attack of opportunity.

Frost Shield. Once per round, the young adult sea dragon can use it's reaction to create a shield of ice from an attack that has half-cover at most and take half damage.

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