In my 5e mashup of Temple of Elemental Evil (Reincarnated) and Princess of the Apocolypse, my PCs ended up having to face Olhydra and the Prophet of Water in the Water Node. I hand-waived a lot of the water node to get the PCs right to the battle; my plan/thinking/expectation was for them to explore the water node if they won against Olhydra as they tried to find their way out. I have 7 PCs and, with NPCs/Henchmen, companions etc., a total party of 12 characters with an average level of 7. So the standard out-of-the-box 5e encounters don’t work. I set up the final confrontation as follows:
- The battle occurs 150 feet below the surface of the water node, forcing the PCs to deal with the water terrain.
- I amped up the restrictions/penalties of fighting underwater to more akin to 3.5e than 5e – details here. PCs could ameliorate some of the issues with magic and equipment, but they didn’t have enough magic to cover everyone to nullify these effects completely.
- I created a more dangerous version of Olhydra – here. I didn’t change her original CR, but she is a very tough CR 18 monster built to take on large groups of PCs.
- Amped up Gar as well to be CR 15 (here) with his own legendary and lair actions that only trigger if Olhydra is defeated/forced back through the portal.
- I gave Gar a mount – a huge shark called Cauldronbubble (inspired by the original Everquest version).
- I added a 1d4 timer that had different effects depending on the situation at the time:
- If Olhydra was still in the water node, the sea level would rise by 10 feet each time the timer counted down to 1. For every 10 feet of rise, the entire party would be subjected to water pressure – 1d6 bludgeoning damage for every 10 feet above 150 the water had risen at the start of their turn.
- If Olhydra had been forced back through the portal, but the portal had not been closed (which is completed by using the weapon Drown on the portal, closing it and destroying the weapon), the timer would show how many rounds before a full-strength Olhydra returns to the water node.
Finally, I added terrain effects beyond it just being underwater. I used miniatures for this battle not Theatre of the Mind:
- 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 player’s 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 centre of the nearest pool. If they are pulled while already in the centre, 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, pulling a PC out without touching it yourself and being subjected to its effects is impossible.
- 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.
In terms of tactics, my plan was to play it by ear mostly with Cauldronbubble using baleful water walk and dispel magic to either remove party members out of the battle temporarily (water walking would shoot them to the surface, and dispel magic will remove water-breathing and freedom of movement) while Olhydra pummeled the PCs and Gar supported with magic. What ended up happening was that Cauldronbubble was dominated in the first round by one of the PCs, and though he didn’t hurt Olhydra or Gar much, taking him out as a threat significantly reduced the threat of the encounter (but good play by my PCs). Cauldronbubble could not break the charm effect, and I didn’t have anything to counteract it.
PCs won in the end with one death (but many more knockouts/near deaths). Once Olhydra was forced back to the elemental plane of water, one of the PCs used telekinesis to take Drown away from Gar and hurl it at the portal, which closed it. This left only Gar on the battlefield which, after 1 round of fighting, he decided to surrender out of despair. In trade for his life and the return of Culdronbubble, he resurrected the fallen PC for the party and accepted self-exile in the water node. My PCs figured he can’t do much harm there without Drown to open a portal. We will see if that’s true over time.