Adapting and Debuffing a Boss Monster from PF to 5e...

[rahimka]

Looking for feedback on this creature I've adapted to 5e from the Pathfinder module "Carrion Hill"

Context/notes:
1) In the original module, this thing is fought with 6negative levels (if the PCs don't screw up). I'm accounting for that by basically inflicting Disadvantage on all its rolls (unless it would have advantage to counteract it)
2) The party facing this thing is 6x lvl5 PCs (fighter, paladin, 2x rogue, druid, ranger), so they've got two decently tanky characters, plenty of damage output, and a huge edge on action economy facing this solo monster (hence the Legendary Actions)
3) "Natural Invisibility" is an amazing asset for this thing, but part of the lead up in the original module is that the party gets warned in advance about this and provided with a scroll to counter it. In this case, the Druid (a pretty new player) will probably also be warned (via an Int check) to prepare Faerie Fire as well

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Spawn of Yog-Sothoth (weakened -> disadvantage on most rolls)
Large aberration, chaotic evil
Armor Class: 16 (natural armor)
Hit Points: 104
Speed: 30ft, climb 30ft
Str 22 (+6) Dex 15 (+2) Con 20 (+5) Int 17 (+3) Wis 17 (+3) Cha 20 (+5)
Saving throws: Wisdom +7, advantage against Magic
Skills: Athletics +10, Intimidate +9, Arcana +7, Perception +7, Stealth +6
Damage Immunities: cold, fire
Damage Resistances: bludgeoning, piercing, and slashing from nonmagical attacks, thunder
Senses: Blindsight 30ft, Darkvision 60ft, passive perception 17
Languages: Aklo
Challenge: 10?
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*Natural Invisibility: this ability is constant, even when attacking. Subject to effects like faerie fire, but cannot be dispelled*
Legendary Resistance (1/day): If the Spawn of Yog-Sothoth fails a saving throw, it can choose to succeed instead
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ACTIONS
Multiattack: The Spawn of Yog-Sothoth makes a bite attack and up to two tentacle attacks, or it makes up to four tentacle attacks. Any of these tentacle attacks can be replaced by Blood Drain, one replacement per tentacle grappling a creature
Bite: Melee Weapon Attack, +10 to hit, reach 10ft, one target, Hit: 9 (1d8+4) piercing damage
Tentacle: Melee Weapon Attack, +10 to hit, reach 10ft, one target. Hit: 7 (1d6+4) bludgeoning damage and the target is grappled (escape DC 14) if it is a Large or smaller creature. Until the grapple ends, the target is restrained and the Spawn of Yog-Sothoth can’t use this same tentacle against other targets. The Spawn of Yog-Sothoth has four tentacles
Blood Drain: One creature grappled by a tentacle must make a DC 16 Constitution saving throw. On a failed save, the target takes 11 (2d10) necrotic damage, and the Spawn of Yog-Sothoth regains half as many hit points
Devastation: The Spawn of Yog-Sothoth can assault a structure, dealing 4d10+12 points of damage to the structure
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LEGENDARY ACTIONS
The Spawn of Yog-Sothoth can take 2 legendary actions, choosing from the options below. Only one legendary action be used at a time and only at the end of another creature’s turn. The Spawn of Yog-Sothoth regains spent legendary actions at the start of its turn
Move: The Spawn of Yog-Sothoth moves up to its speed
Bite Attack: makes one bite attack against a creature it has grappled
Tentacle Attack: attacks one creature within its reach using a tentacle attack

[Kane0]

With a rough eyeball yeah it looks okay. Does anyone in the party have a magic weapon?

You may want to check its stats/saves, not much chance of a level 5 spell DC getting through.

Edit: instead of disadvantage to simulate the negative levels how about some exhaustion?

[rahimka]

With a rough eyeball yeah it looks okay. Does anyone in the party have a magic weapon?

You may want to check its stats/saves, not much chance of a level 5 spell DC getting through.

Edit: instead of disadvantage to simulate the negative levels how about some exhaustion?

Thanks for the feedback!

The Fighter, Ranger, and the Rogue-Swashbuckler all have magic weapons. The Paladin doesn't yet, but he can still get some Radiant damage through with Smites and I can be sure to at least HINT at the physical damage resistance when they get the chance to learn about the Spawn pre-fight (give him the same Int check the Druid gets to suggest prepping Magic Weapon).

As far as the saves, the original PF statblock had a Spell Resistance of 20(!), so it really is meant to be a b*tch for direct magic to get through against it. In any case, the party Druid is very Fire focused (homebrew archetype based on the PC being a pyro at low levels), so she's gonna have a hard time directly going after this thing no matter what. She's already learned to use Conjure Beasts and support casting in those scenarios pretty effectively though. Between that and Faerie Fire (the Spawn has a pretty bad Dex save), I think she'll still feel pretty useful.

Exhaustion isn't a bad idea. Looking at the various levels, the only real difference between what I was already planning (Disadvantage on abilities, attacks, and saves) and 3lvls of exhaustion is that it would also get its speed halved. I think that might be overkill in this case, letting the PCs just kite the thing once they tag with Faerie Fire. But I could reflect that by dropping the Move option among the Legendary actions. Definitely something to keep in mind for any other adaptions that come up involving negative levels though. THanks!