Actions

Engulfing Wave. Olhydra makes two Wave Slam attacks or two water jet attacks. She gains an additional attack when she is bloodied.   Wave Slam. Melee Weapon Attack: 1d20+11 to hit, reach 10 ft., one target. Hit: 21 ( 3d10+5 ) bludgeoning damage, and the target is grappled (escape DC 19). Olhydra can grapple up to four targets. When Olhydra moves, all creatures she is grappling move with her. Creatures adjacent to the target (within 5 feet) are pushed back 30 feet and knocked prone on a failed DC 19 Strength saving throw. On a natural 1 the target must spend all their movement to stand up.   Call of the Sea. Ranged Weapon Attack: 1d20+12 to hit, range 120 ft., one target. Hit: 21 ( 6d6 ) bludgeoning damage, and the target is knocked prone if it fails a DC 19 Strength saving throw. On a natural 1 the target must spend all their movement to stand up.   Summon Elementals (1/Day). Olhydra summons up to three Water Elementals and loses 30 hit points for each elemental she summons. Summoned elementals have maximum hit points, appear within 100 feet of Olhydra, and disappear if Olhydra is reduced to 0 hit points. She can also re-absorb these elementals at the start of her turn if they are adjacent to her and she heals for 30 hit points.

Reactions

Instinctive Flood (1/Day). When Olhydra is first bloodied any negative condition on Olhydra ends. She moves up to her speed. During the move, she can move through enemies' spaces, she has resistance to all damage, and she can't be hindered or stopped from moving by any attack or effect. Each time she enters an enemy's space for the first time during the move, she uses Wave Slam against that enemy.

Legendary Actions

Olhydra 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. Olhydra regains spent legendary actions at the start of her turn.

• Crush. All creatures that Olhydra is grappling is crushed for 21 ( 3d10+5 ) bludgeoning damage.
• Fling (Costs 2 Actions). Olhydra releases one (or more) creature she is grappling by flinging the creature up to 60 feet away from her, in a direction of her choice. If the flung creature comes into contact with a solid surface, such as a wall or floor, the creature takes 1d6 bludgeoning damage for every 10 feet it was flung.
• Water to Acid (Costs 3 Actions). Olhydra transforms her watery body into acid. This effect lasts until Olhydra’s next turn. Any creature that comes into contact with Olhydra or hits her with a melee attack while standing within 5 feet of her takes 11 ( 2d10 ) acid damage. Any creature grappled by Olhydra takes 22 ( 4d10 ) acid damage at the start of its turn.

Lair Actions

Olhydra can command the waters around herself to take lair actions on the Elemental Plane of Water, or in any elemental water node (such as the Plunging Torrents, described in chapter 5). On initiative count 20 (losing initiative ties), Olhydra uses her lair action to cause one of the following effects:

• Pools of water in the lair surge outward in a grasping tide. Any creature within 20 feet of such a pool must succeed on a DC 20 Strength saving throw or be pulled up to 20 feet into the water and knocked prone.
• Water within 120 feet of Olhydra becomes murky and opaque until initiative count 20 of the next round. A creature with darkvision can’t see through the water, and light can’t illuminate it.
• A freezing fog fills a 40-foot-radius sphere within 120 feet of Olhydra, lasting until initiative count 20 of the next round. Creatures and objects within or beyond the fog are heavily obscured. A creature that enters the freezing cloud for the first time on a turn or starts its turn there takes 10 ( 3d6 ) cold damage.

The Princess of Evil Water is native to the Elemental Plane of Water, where she can be found in the black depths of vast seas or enthroned among jagged reefs. Olhydra waits for the chance to enter the Material Plane through elemental water nodes or when called by the proper rituals. Whether she is in her elemental domain or temporarily occupying a water node, Olhydra commands the waters around her and can shape them to her will.

LAIR TERRAIN

These affects are shown on the tactical map.   Whirlpools. When Olhydra enters the water node two stationary whirlpools are at the edges of the room, 60 ft from the players (positioned so the Princess is ahead, and whirlpools are directly left and right at the edges). This triggers at the start of each players turn. If they swim towards the whirlpool, they have disadvantage on their saving throws. All characters must make a DC 14 athletics/acrobatics check or be pulled 10 ft towards the center of the nearest pool. If they are pulled while already in the center, they take 3d8 cold damage, halved by a DC 14 Con save.   Seaweed paralysis. Unnaturally giant kelp rises from the ocean floor like a forest dotting the combat area. A DC 12 Investigation/Perception check reveals eels living amongst the kelp, but other dead creatures (humanoid, fish, or otherwise) wrapped up in some of the kelp. When a PC makes contact with the kelp they must make a DC 15 Con Save and are incapacitated upon failure. The kelp will then start to wrap around the incapacitated PC. After 1 round, an incapacitated PC still in contact with the kelp makes another DC 15 Con Save and is paralyzed upon failure. After 1 more round, a paralyzed PC makes a final DC 15 Con Save and is unconscious upon failure. They then begin making death saves. A PC with immunity to poison is unaffected by the kelp. Worshipers of Olhydra are immune to this affect.   A PC can easily cut the kelp (8 AC and 1 HP - immune to piercing and bludgeoning), but it is impossible to cut the kelp in melee without touching the kelp due to its floppy nature. A PC with a slashing weapon with reach might be able to cut the kelp without touching it. A PC wrapped up in the kelp can be easily torn out with a DC 10 Athletics check, however it is impossible to pull a PC out without touching the kelp yourself and being subjected to its effects.   Turbulence. This water swirls about madly; anyone entering the zone must make a DC 22 Reflex save or be flung 1d6×10 feet in a random direction.   Thick Water. The water here is thick and sluggish, so moving through this area is like swimming through pudding. A character entering one of these zones must stop and can move no farther in this round. In the following round he must make a DC 12 Strength check to leave the thick water and even then can move at only half speed in that round. On a failure, the character is stuck and cannot move, but can take any other action.  

Timer

If Olhydra is defeated but Gar Shatterkeel is still alive, Olhydra will return in 1d4 rounds, as the way from her palace to the water node remains open until drown is used to close the portal. She returns full strength.

Regional Effects

Olhydra’s presence in an elemental node creates a number of widespread effects — bad weather, strong tides, and increasing aggression from dangerous sea life.

• Violent downpours become frequent within 10 miles of the lair. A downpour occurs once every 2d12 hours, and lasts 1d3 hours. Downpours are so heavy that creatures moving overland travel at half normal speed.
• Within 5 miles of the lair, currents and tides are exceptionally strong and treacherous. Any ability checks made to safely navigate or control a vessel moving through these waters has disadvantage.
• Aquatic creatures that have an Intelligence score of 2 or lower within 1 mile of the lair must succeed on a DC 15 Wisdom saving throw when they enter the area. If the saving throw succeeds, the creature is frightened and attempts to leave the area, remaining frightened while within it. On a failure, the creature becomes highly aggressive and remains in the area for 24 hours. While in this state, the creature gains advantage on saving throws against charm and fear effects.
• Natural springs and pools within 1 mile of the lair form intermittent portals to the Elemental Plane of Water, allowing elemental creatures into the mortal world to dwell near those points.

If Olhydra is destroyed or banished back to her home plane, the regional effects fade over the next 1d10 days.

Suggested Environments

Underwater, Elemental Plane of Water