Traits

Amphibious. Breathes both water and air.

Siege Body. Deals double damage to objects and structures.

Whirlpool Aura. While the Leviathan Warden is in water, the water counts as Difficult Terrain for every enemy within 20 feet, and that enemy makes a DC 19 Strength save when it ends its turn. Failure: It is pulled 15 feet toward the Leviathan Warden.

Warden's Reach. Enemies within 20 feet of the Leviathan Warden provoke an Opportunity Attack when they move away from it.

Actions

Multiattack. Makes one Crushing Bite and two Tendril attacks.

Crushing Bite. Melee Attack Roll: +14, reach 15 ft. Hit: 30 (4d10 + 8) Piercing damage. If the target is Large or smaller it makes a DC 22 Strength save. Failure: The target is swallowed — it becomes Blinded and Restrained, has Total Cover against attacks from outside, and takes 21 (6d6) Acid damage at the start of the Leviathan Warden's turn.

A swallowed creature can escape by spending an action and making a DC 22 Strength (Athletics) check. The Leviathan Warden can keep two creatures swallowed at once.

Tendril. Melee Attack Roll: +14, reach 25 ft. Hit: 21 (3d8 + 8) Bludgeoning damage, and the target makes a DC 22 Strength save. Failure: The target becomes Grappled (escape DC 22) and is pulled 20 feet toward the Leviathan Warden.

Tidal Slam (Recharge 5–6). The Leviathan Warden swings its body. Every creature in a 40-foot Emanation makes a DC 20 Dexterity save.

• Failure: 45 (10d8) Bludgeoning damage, flung back 30 feet, and becomes Prone.
• Success: Half damage, not flung.

Reactions

Warden's Bulwark (2/Turn). When an enemy within 25 feet of the Leviathan Warden attacks a target other than the Leviathan Warden or moves away from it: the Leviathan Warden makes a Tendril attack.

How to Use

CR 15, and Tier 4's first true Soldier.

Whirlpool Aura is the core mechanic: the party is constantly pulled toward it. They can't get away. The water brings them back.

Reach of 25 feet (Tendril) — very long. Even a Rogue trying to attack from range gets caught.

Crushing Bite swallows the target — Total Cover, ongoing Acid damage, and escaping requires DC 22 Athletics (very hard).

Warden's Bulwark (2/Turn) — anyone who ignores it and attacks someone else gets punished twice.

The underwater combat rules also work against the party:

A creature without a Swim Speed has Disadvantage in melee (except Piercing). Ranged attacks automatically miss beyond normal range. It gains Resistance to fire.

So the party is already at a disadvantage. The Leviathan Warden exploits this.

How the party wins:

• Piercing weapons — don't suffer Disadvantage underwater
• Water Breathing + Freedom of Movement — the latter completely negates Whirlpool Aura and being Grappled. This turns the fight around.
• Luring it onto land — Speed 20 ft. on land, Whirlpool Aura doesn't work. But this is hard.
• Force/Radiant damage — not on its Resistance list.

Combination (Underwater Boss):

Kraken (Boss) + Leviathan Warden

The Warden holds the party, the Kraken pounds them. Classic Soldier + Boss.

Balance note: The DC 22 escape check is very high — a Lv15 Fighter (Athletics +10) succeeds 45% of the time. This is intentional: being swallowed should be a serious problem. But it's limited to two people, and they're freed when the Leviathan Warden dies.