Concern about an upcoming planned battle: 1 Shambling mound + 4-8 Will o wisps

[D&DPrinceTandem]

So the party (collective CL 15) is going to be soon passing through a swamp area. In Character they have already been warned the dangerous spirits and monsters haunt the eastern swamp lands that they are going to pass through, one of the characters (a ranger) rolled a History and rolled high enough to know that the Eastern swamp lands goes by another name more fitting, "The Souls Light Swamps". and he knows this is more fitting because it is known that the swamps are filled with countless Will o' wisps. He also rolled a Nature check, high enough to know that the shambling mounds they fought while entering the tribal town only grow more common the further into the jungle, and by proxy the swamps, they go. So they have gained enough information to expect at least one encounter containing each of the creatures. I read there stat blocks and info before telling them this information and thought it would be a nice challenge. Now I grow worried for the campaign.

The written battle consists of 1 Shambling mound and 4-8 Will o' wisps. (anywhere from a CR 15 battle to a CR 11 battle). I knew it was going to be challenging and now I am concerned with exactly how challenging it would be. Clarification: the party hasn't reach the upcoming battle yet.

If i am reading correctly here are a few things i am worried about.
(a) the Wisps can travel at a whopping speed of 50ft. This is enough to travel 25ft to the shambling mound, make a shock attack, and move 25ft away.
(b) the Wisps don't need to breath.
(c) 4 wisps hit the Shambling mound healing it on average 36 dmg (8 = 72 average)
(d) Shambling mounds have 208hp (we max all hp for everything)
(e) Shambling mounds moves at 20ft in water (and with this it can hold its breath for 4 minutes)
(f) Shambling mounds have Blindsight 60ft
(g) a creature engulfed by a shambling mound can't breath and drops to 0hp and is dying in a number of rounds = to its con mod
(h) the river/swamp is 40ft deep and murk water (10ft visibility).

This is the planned tactics of the wisps
Go 25ft under the water (they don't need to breath); on there turn come up and touch the bottom of the Shambling mound healing in ~9dmg, ~36-72 dmg a round. If the shambling mound is at full health and a character is engulfed they will attack the character (which they have advantage on).

and here is the planned tactics of the Shambling mound
Attack, a single creature twice. if success the target is grappled and begins to suffocate. Then the mound moves 20ft underwater to escape other creatures. after 5 rounds it rises back to the surface, the party 1 member down and the shambling mound back at what should be full heal no mater how much damage was dealt to it (healed 360 per round)
What do you think U should do? I would let the players down as a DM if I don't included these creatures in a battle. The Players at the start of the campaign have already been warned the this world is a harsh place (such the reason why there are few Adventures). I am okay with killing one player as that would be expected from this level of a campaign, but how do i prevent this from becoming a TPK (because straight numbers, it most likely will be).

[Belthien]

(g) a creature engulfed by a shambling mound can't breath and drops to 0hp and is dying in a number of rounds = to its con mod


The rules for holding your breath are on page 183 of the PHB. A creature can hold its breath for a number of minutes equal to 1 + its Constitution modifier (minimum of 30 seconds). So, even if someone only has a +0 con bonus, they can still hold their breath for 10 rounds.

People will probably die from the start of turn damage from being in the stomach before breath becomes an issue.

[LuccMa]

Since you are concerned, here are some reasons to make it easier on your players:

1. You can rule that the Wisps actually have to hit the mound (+4 to attack) or if he drops his guard so the Wisps can hit him, the players gain advantage or auto-hits. I would prefer to roll for the Wisps, though.

2. Wisps as i see it have no swimming speed (just hovering). I would rule they cant just dash 50 feet in and out of water as it is difficult terrain even for them. That means your tactic only works if everything takes place in that 40ft river. See point 3.

3. Maybe the swamp area where the fight takes place doesnt actually have 40ft deep water everywhere - somehow the PCs need to walk on something if they arent travelling by boat or other means, right? Limit the space the Wisps can "hide" after healing the Mound. If the PCs fight the mound + wisps IN a 40ft deep river that would be really stupid - they should stick to safer ground, so make them some safe ground available. Saying "Yeah, the terrain i chose is really bad for you. Sorry." without offering them a way to evade that terrain is a ****ty move :D

This whole encounter sounds great, to be honest. As soon as the PCs realize those Wisps are just healing the mound back up, it gets really messy and they have to figure out how to survive that. This great thematic of the fight works just as well in a swamp where everyone can walk and there is not THAT much deep water around the encounter.
When they realize that they might want to flee, make it possible. Surviving a hard encounter by running can be just as satisfiyng. The whole underwater thing just feels like an additional penalty for a otherwise already hard encounter and i dont think it needs it.

Kind regards,
LuccMa

[Chronos]

Wisps fly by being lighter than air. I'd expect that it'd be completely impossible for such a creature to go underwater, not just difficult terrain. CORRECTION: Apparently they've changed the fluff on them, and now they're incorporeal undead, not gaseous aberrations, so maybe they can go underwater, but it's still difficult terrain.

Shambling mounds aren't very intelligent. They won't use any tactics more complicated than what's described in their Monster Manual entry.

Wisps are intelligent enough to come up with working with a shambling mound, and the mound is at least smart enough to realize that it likes these things hanging around. So there's still reason for the encounter.

Wisps don't have very many HP, and the party will probably figure out they need to kill the wisps first, but they resist everything but radiant, psychic, force, and magic weapons. Does the party know this, and do they have ways of doing this?

One effective tactic I see (which the party might use even if they don't know what works against what) would be for a cleric to use Spiritual Guardians and get right up next to the mound. Upcast to 5th level or so, and the wisps will probably be killed in two shots each, while also dealing damage against the mound. Or rather, the wisps will stay away from the cleric, and hence be unable to shock the mound.

[Zanthy1]

I'm going to echo most of what Chronos said. Also throw in that if this is a swamp, then much of the terrain should be a mixture of water and mud, allowing for paths and even clearings large enough for a party to make a camp and such. If they're diving into the water to fight this thing thats their decision to get shrekted, but Ideally they're gonna keep far enough away that this guy needs to come on land.

As a DM I would make sure that there are multiple locations where the PCs can stand without being underwater and being able to be effective in this combat, but I would only limit the wisps to not being able to go underwater. Everything else seems fine.

Side note: If the party seems to be getting overwhelmed, have an Ogre come a rescue them from the Shambling Mound and then kick them out of his swamp for trespassing

[Darkstar952]

Do any of the players have chill touch? because once they figure out the wisps are healing the mound they will then be able to shut that tactic down.

I agree with the others that the terrain needs to be a mix of muddy land and water and shouldn't be forced to fight in only the water.

[redwizard007]

This sounds like an amazing encounter, and I will definitely be stealing some inspiration.

I do think it sounds a little rough as planned. The earlier suggestions to limit water depth/add more dry land could help mitigate that. So would an earlier encounter with a shambling mound and a single wisp. If the PCs have seen the tactic, and been given clues that the wisp was healing the mound, then I would have no issues dumping them in a deeper mire with less dry land as a later encounter. You would be allowing character knowledge to offset the poor terrain, that's entirely fair.

[Sigreid]

I think you get to find out if your players are savvy enough to go after the healers first.