Scaling up a hydra for tonight's session (help!)

[Mongrel]

So I procrastinated a bit on my campaign and long story short I could use some help balancing the final encounter.

My party consists of 5 level 7 PCs (a cleric, two rogues, a bow ranger, and a paladin). The PCs will also have a fairly weak NPC with them (no class levels, 20 hp, wields a +1 warhammer poorly and a wand of magic missile, has some healing potions and antidotes), and they will be encountering another friendly NPC (level 7 fey warlock), though it's possible they'll be too suspicious of him and he won't be present for the final battle. No one is optimized (NPCs included), but they're able to customize their magic item loadout from an armory of sorts before starting the dungeon, and have a few boons and whatnot.

They're going to a poison themed Aztec-ey pyramid to retrieve an artifact at the apex. I'm planning to essentially spawn a hydra on them once they take the artifact (a little more complicated than that, but you get the idea). The way the encounter is designed it will be tough for them to flee. I just want to make sure I don't accidentally wreck them, but I also don't want the encounter to be a pushover.

Using Kobold Fight Club, it appears that 2 hydras is a Deadly encounter for 5 level 7 PCs, but only hard for 6 level 7 PCs (which is what it would be if they take the NPC warlock along with them). I was thinking that I'd just make it one ten headed hydra (rather than two five headed ones) to essentially make it harder for them, though I also want to add a poison effect to the hydra's attacks to bring it on theme with the rest of the dungeon. I'm really at a loss for how exactly to scale this to be a reasonable encounter though (i.e. how many heads should there necessarily be, how much does making a ten headed hydra vs two five headed ones add to CR, what an appropriate poison effect to add on would be considering how many attacks a hydra gets, etc.). Specifically, here are the questions that are mostly coming up for me:

1) What would be a fair poison effect to add on to the hydra's attacks? Serpent Venom seems appropriate; that adds 3d6 damage to the attack (half on successful DC 11 con save), is that too high? What about adding the poisoned condition instead of (or in addition to) extra damage (negated by a successful save ofc), is that too brutal?

2) How should this be changed on the spot with fewer PCs? If, for instance, one of my players cancels at the last moment, or if the party kills the NPC warlock?

3) How many heads should the hydra actually have, especially if poison is added to their attacks? This might be the trickiest issue I'm looking at, since a hydra's advantage seems to be its number of attacks; it only does 10 damage on average per hit using the base stats, adding additional damage or a poison debuff onto that seems like it could multiply the creature's power more than I intend.

I expect that the party will be fully rested before starting this encounter. Any advice would be nice. Currently I'm thinking of making each head do an extra 1d6 or 2d6 damage and add the poisoned ailment (DC 13 con save for half damage and no debuff), does this seem reasonable? If not, what would be better?

The game is tonight, so quick responses are appreciated lol.

[stoutstien]

Skip to poison and just have a the hydra be in a body of water. Hydras are not smart but they can use hit n run tactics(low int but ok Wis)
It would also lessen the impact of fire attacks. I've found blocking the party's plan A is great for increasing difficulty but never block plan b.

So have the bulk of the Hydra under water and have a few heads come out and attack.
Side note it could replace a attack with a grapple check to make it a more dynamic encounter.

[Derpaligtr]

Skip to poison and just have a the hydra be in a body of water. Hydras are not smart but they can use hit n run tactics(low int but ok Wis)
It would also lessen the impact of fire attacks. I've found blocking the party's plan A is great for increasing difficulty but never block plan b.

So have the bulk of the Hydra under water and have a few heads come out and attack.
Side note it could replace a attack with a grapple check to make it a more dynamic encounter.

Have the hydra be the body of water and some properties of a water elemental.

[Shining Wrath]

Hydras in water is a natural.
If you want poison, I suggest that only some of the heads are poisonous, and they are a different color.

[Mongrel]

Skip to poison and just have a the hydra be in a body of water. Hydras are not smart but they can use hit n run tactics(low int but ok Wis)
It would also lessen the impact of fire attacks. I've found blocking the party's plan A is great for increasing difficulty but never block plan b.

So have the bulk of the Hydra under water and have a few heads come out and attack.
Side note it could replace a attack with a grapple check to make it a more dynamic encounter.

Hm, well just to give a little more info on the encounter, the players are going to encounter the creature in a relatively small room where the only exit is essentially a slow moving elevator. I was planning on having the hydra heads pop out of treasure which coats the floor of the room once they remove the artifact (which is on a pedestal at the back of the room, with a catwalk of sorts leading from the elevator to the pedestal). Pretty similar to being underwater, doesn't get the mobility but the room is small enough to where that doesn't really matter. Only the heads will be seen so they can't really attack the body (though the way hydras work in 5e it doesn't seem to matter if they attack the body or the heads)



Hydras in water is a natural.
If you want poison, I suggest that only some of the heads are poisonous, and they are a different color.

I considered this, and it could work, though I'd sort of prefer that all the heads are poisonous if it can be done. The creature is essentially supposed to be the pet of the green dragonborn queen who was entombed here, bred specifically to be poisonous given the queen's propensity for it. It just seems cleaner if the whole thing is poisonous to me.

If it's easier to balance, I could try to just use one five headed hydra and soup it up (with poison damage/debuff and maybe a spitting attack or even breath weapon or something)

[the_brazenburn]

I'd advise looking at the DMG challenge rating tables to determine how much you need to increase the damage and HP, then do so. You can fluff part of the damage as poison, or give it a rider like:

Poison Body: When 10 or more damage is dealt to the hydra by a single source, every creature within 10 feet must make a DC 12 Dexterity saving throw or take 10 (3d6) poison damage.

[Mongrel]

I'd advise looking at the DMG challenge rating tables to determine how much you need to increase the damage and HP, then do so. You can fluff part of the damage as poison, or give it a rider like:

Poison Body: When 10 or more damage is dealt to the hydra by a single source, every creature within 10 feet must make a DC 12 Dexterity saving throw or take 10 (3d6) poison damage.

Indeed, I've been fiddling around with the DMG rules for modifying monsters (starting on p.273), and here's what I've come up with:

I'd like the challenge rating for the encounter to be somewhere in the 12-18 CR range. Large range, I know, apparently CR 13 is deadly for 5 level 7 PCs, but if you have 6 level 7 PCs it doesn't get deadly until CR 19. I figure, then, it would be advantageous to play around with a few different encounter CRs depending on the number of players who come and how much I feel like challenging them.

Here's the (ostensibly) CR14-ish encounter:

one seven headed hydra. HP upped from 172 to 265 and AC upped to 17 (as per the scaling chart on p.274 of the DMG; actually 1 hp lower than the minimum for a CR 14 monster and 1 AC lower than the suggested AC, but I figure the hydra head regeneration thing warrants some hits to its defense). Dex improved to 16 (hence the increase in AC, though this also adds to dex saves) Bite attacks do an additional 1d6 poison damage but don't inflict poisoned condition (con save DC 15 for half damage; apparently saves for CR 14 monsters should be DC 18 according to the chart so this seems relatively generous), but otherwise unchanged (same to hit bonus). New attack: poison spit (single target, +8 to hit, 3d6 poison damage and target gains poisoned condition; DC 15 con save halves damage and negates condition). The hydra will also be resistant to poison, though I don't foresee this coming up as the PCs rarely use poison.

Going off the bite attack, this is 13 average damage per attack (11/12 if saving throw is made) * 7 attacks = 91 (77/84) damage/round (suggested for CR 14 is 87-92). Note that there's the potential for more heads to form, which obviously increases the damage potential.

Now, for 6 level 7 PCs, CR 14 is only a Medium encounter. It starts getting Hard around CR 16, and Deadly at CR 19. I dunno if I want to throw a CR 19 encounter at them as that seems like it might be too much no matter what Kobold Fight Club might say, but perhaps the CR 14 one would be too easy. To scale the encounter up to CR 17, I'm thinking I'd increase the damage on the bite to 2d6 (save for half) OR add the poisoned condition to the bite. If increasing the damage, that brings the average to 112 if not saved and 91 if saved (suggested per round damage for CR 17 is 105-110). The poison spit attack damage could also be increased to 4d6+2 (average per round of about 91 or 49 if save; lower than suggested for the CR though the threat of poisoned condition seems to make this powerful enough). The HP and AC should also be increased accordingly (to 310 and 18 respectively)

Which one of the above statlines looks good? Is the CR 14 too easy, the CR 17 too hard? I could see potentially increasing the save DCs for its poison, possibly to 18 as suggested (maybe even 19 for the CR 17)? Or is that a little too high? Also consider: they will have one other NPC with them that I didn't factor into the difficulty calculations: the guy with no levels, a +1 warhammer, and a wand of magic missiles. Action economy wise this might give them an edge, but it also could be a detriment since he should be pretty easy to down, and he's a love interest for one of the PCs, so it's unlikely they'd just let him die.

If there are only 5 PCs (they off the warlock NPC, or a player doesn't show), taking the CR 14 statblock, removing a head, and adding +2 damage to the poison attacks while decreasing the HP and AC to 250 and 15 respectively and keeping everything else constant seems to be a good fix, as this appears to give it around a CR 13.

If only four level 7 PCs, decreasing the heads to 5 and using the initial stats (the CR 14 one) while lowering AC and HP accordingly (to 220 and 15 respectively) seems to be around a CR 10/11 encounter, appropriate for their level.

Do these calculations look good? Math isn't exactly my strong suit, and I'd hate to throw something too hard (or not hard enough) at the party due to a math error on my part.

[Fredaintdead]

Okay, so, I ran the maths through the DMG's "Create your own monster" thing.

Defensive:
Base HP of 265 + Multiple Heads (10 healed per head gained, given that your party doesn't LOOK like it has particular access to fire damage, I'd estimate that's worth a Regeneration of 20, which according to the DMG adds 60 effective hp) leaves us at 325hp (CR 17), which then drops by 1, as AC 17 is 2 below AC 19 (the estimated AC for a CR 17 monster).

Final result: Defensive CR 16.

Offensive:
Like you said, the damage is roughly 91 per round. It's a little modular (in that damage output correlates to number of heads, and that can fluctuate), but that fits in CR 14. Usual attack bonus for that CR is +8, based on where we're looking at in terms of CR, it'd have proficiency +5, and with Str 20, that's a to-hit of +10, so offensive CR goes up by 1.

Final result: Defensive CR 15.

Final CR = (16+15)/2 = 15.5. Rounds up to 16.

So the "CR 14" version you've got there is actually CR 16 (primarily due to its regenerative abilities, which potentially add a fair chunk of extra hit points). And honestly, that's probably fine. Based on having a Paladin and 2 Rogues, it looks like you've got a party that can perform some pretty impressive damage (the Paladin can produce some surprising burst in particular), so I'd consider giving the Hydra things that it can do on other peoples turns. Might I suggest the following in a watery environment (also the name Venohydra, because Latin and I like the idea of elemental Hydras):

Legendary Actions: The Venohydra 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. The Venohydra regains spent legendary actions at the start of its turn.
Bite (1 action): The Venohydra makes a single bite attack against an enemy within melee range.
Spit Venom (1 action): The Venohydra "casts" Poison Spray as a 11th level caster (3d12 poison damage). Con DC 18.
Submerge (1 action): The Venohydra disappears into the water, ending the Restrained condition on itself if present. The Venohydra reappears at a body of liquid within 30ft.

The idea being that the Hydra can still be a threat outside of its own turn, and has a movement option that don't quite let it hit-and-run, but do make the fight a little more interesting. In any case, this sounds like a super rad fight.

[Mongrel]

Okay, so, I ran the maths through the DMG's "Create your own monster" thing.

Defensive:
Base HP of 265 + Multiple Heads (10 healed per head gained, given that your party doesn't LOOK like it has particular access to fire damage, I'd estimate that's worth a Regeneration of 20, which according to the DMG adds 60 effective hp) leaves us at 325hp (CR 17), which then drops by 1, as AC 17 is 2 below AC 19 (the estimated AC for a CR 17 monster).

Final result: Defensive CR 16.

Offensive:
Like you said, the damage is roughly 91 per round. It's a little modular (in that damage output correlates to number of heads, and that can fluctuate), but that fits in CR 14. Usual attack bonus for that CR is +8, based on where we're looking at in terms of CR, it'd have proficiency +5, and with Str 20, that's a to-hit of +10, so offensive CR goes up by 1.

Final result: Defensive CR 15.

Final CR = (16+15)/2 = 15.5. Rounds up to 16.

So the "CR 14" version you've got there is actually CR 16 (primarily due to its regenerative abilities, which potentially add a fair chunk of extra hit points). And honestly, that's probably fine. Based on having a Paladin and 2 Rogues, it looks like you've got a party that can perform some pretty impressive damage (the Paladin can produce some surprising burst in particular), so I'd consider giving the Hydra things that it can do on other peoples turns. Might I suggest the following in a watery environment (also the name Venohydra, because Latin and I like the idea of elemental Hydras):

Legendary Actions: The Venohydra 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. The Venohydra regains spent legendary actions at the start of its turn.
Bite (1 action): The Venohydra makes a single bite attack against an enemy within melee range.
Spit Venom (1 action): The Venohydra "casts" Poison Spray as a 11th level caster (3d12 poison damage). Con DC 18.
Submerge (1 action): The Venohydra disappears into the water, ending the Restrained condition on itself if present. The Venohydra reappears at a body of liquid within 30ft.

The idea being that the Hydra can still be a threat outside of its own turn, and has a movement option that don't quite let it hit-and-run, but do make the fight a little more interesting. In any case, this sounds like a super rad fight.

Heya, thanks for crunching the numbers for me! =D

So just to make sure I understand, you're saying that I should use my "CR 14" (actually closer to CR 16) statline, but add some legendary actions, correct? Just to be clear, were you suggesting I replace the "poison spit" attack with the "spit venom" legendary action, or just add on that legendary action while keeping the "poison spit" attack in addition? Also, are you suggesting the to-hit bonus should be +10 per attack rather than the +8 I originally had? Seems very scary given the amount of attacks the hydra gets (one per head plus a number of opportunity attacks equal to number of heads minus one), but if that's what the book says I'll probably do it; I've had encounters I stressed out about being too tough end up being pushovers in the past, so I think I tend to err too much on the side of caution lol. Furthermore, should the DCs for the poison damage on the attacks be upped to 18 then, or kept at 15?

As for the watery environment, I'm not sure it works too well at the apex of the pyramid, but I could just have the hydra "swim" through the treasure that's there...or hey, maybe some of the treasure will be an illusion over water >=D

As for the fire damage, the cleric of the party actually just got a boon that ups his fire spells and allows him to cast Sacred Flame to do fire damage rather than radiant, so they have a little more fire than your standard party. He's really the only one who has access to fire I believe, unless the ranger has some spells to that end but I don't recall him ever using any. The rogues and paladin don't, unless they end up bringing alchemist's fire (which they have done in the past, so we'll see)

Also, Venohydra is a perfect name, totally using that ;)