Quickleaf
Legend
Devourer (AC 16, hp 178) has no special defenses – so your party will focus fire and crush it fast, unless you focus on some creative defenses for it. Rather than boosting numbers, I prefer to make the special defenses unique to my specific scenario (e.g. a mighty lich with multiple phylacteries of bound liches that it can expend to work like a boosted Legendary Resistance).
For starters, I'd begin with the devourer having an NPC as its Imprisoned Soul - this gives the PCs a "friendly fire" dilemma, and depending how you set it up can give the devourer a HP boost.
I'd consider giving the devourer a reaction: Malign Transfer: The devourer transfers damage or one condition from an attack or spell from itself to either its Imprisoned Soul or a zombie/ghoul/wight created by Imprison Soul. This gives you some encounter building play with "how many of these undead I'm including were created by the devourer's Imprison Soul already?"
Also the devourer is exclusively a melee / close range enemy. A longbow PC or Sorcerer could dish out several rounds of damage before the devourer could close the gap. This means you want to do at least one of the following: (1) give it a ranged power, (2) give it a way to teleport/move rapidly to close the gap, (3) give it a way to hide from ranged spells/attacks or mitigate ranged spells/attacks.
For added nastiness, note that Soul Rend becomes deadlier the more weak humanoids are nearby and at 0 hp... so adding weak militia, innocents, sacrifices... those can all make this encounter nastier.
Lastly, one thing I did when I ran a devourer was that not only could the devourer use its telepathy, but the soul trapped inside of it could use telepathy too, as a sort of "pull on their heartstrings" / inject some roleplay into combat.
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I'm reorganizing your quotes to cluster similar concepts together, to make replying easier, and my replies will be in bulletpoint moves you can make... big theme you'll notice in my replies are "soft counters" ("yes, but")...
- Time-delayed mooks: Waves of mooks – Force/glassteel barriers – Some mooks are illusory
- Damage type hijinx: Mooks that transform or triggersomething when taking necrotic or radiant damage (e.g. spirit guardians) – Mooks with retaliatory effects like psychic transference
- Targeting complications: Distance distortion effects – Innocents or fragile McGuffins in the midst of the mooks – No normal line of sight until almost on top of them (e.g. fog)
- Non-humanoids: Undead can't be targeted by hold person and are unaffected by command – Human mooks enchanted by "veil of undeath" giving them temporary undead creature type
- Tricks: Language barriers (or deliberately imposed language barriers such as deafening or silencing) – Monster taking an ongoing action which, if stopped by command, could have deleterious consequences for PCs – enemy pretending to be paralyzed/charmed
- Better Saves: Fey Ancestry / Magic Resistance – Wis save bolstering leader-type effect
- Turnabout is fair play: having an enemy caster using the command or hold person spell on PCs
- Two-in-one monsters: Gelatinous cube with a golem inside (or golem with ooze inside) – Animated armor operated by a mephit – Zombie infested by rot grubs – Mage possessed by a ghost (body gets paralyzed, then second monster escapes)
- Spellcatching: (reaction) monster gets +4 AC versus a ranged spell attack, and if the attack misses, they may reflect it back at the caster or a creature within 5 feet of caster, rerolling attack
- Wild magic zone: the zone randomly determines damage type of variable spells like chromatic orb
- Prismatic arcana: prism/mirror-like distortion of space making any prismatic/chromatic/rainbow spells into 1d6 copies in randomly determined spaces
- Oopsie daisy: Big monsters that fall prone break through the floor, opening new dungeon, and maybe risks dropping everyone into it – Monster falling prone triggers a trap affecting the PCs as well
- HP to shake off: Monster can spend X hit points to shake off a condition like stunned (10% of max HP is a guideline I've used in the past)
- Makes it more dangerous: While stunned / incapacitated, the monster erupts in a necrotic version of fire shield – While unable to the move, the monster radiates a growing aura of shadowy whispers that acts like an area effect dissonant whispers
- Condition mitigation:
- If the elite would be stunned or dominated, the elite instead grants advantage to all attacks made against it and suffers vulnerability against the next attack to hit it before the start of its next turn, and the elite must repeat the saving throw at the start of its next turn; if it succeeds it continues to grant advantage for as long as the spell or effect dictates it would be stunned or dominated, whereas if it fails it becomes stunned or dominated as normal.
- Hazards of bright light: Medusa in a dark room – Lurking monsters like sorrowsworn or vampiric moths that are attracted / hostile to bright light – Glow-in-the-dark trapped spell glyphs that cannot be seen in bright light conditions
- Focus fire: (used sparingly) "Stab out the eyes of the enemy of darkness" – Gargoyle head that spews darkbolts (necrotic magic missiles) at any caster with ongoing light magic or creature enchanted with light magic/glowing magic item
- Weird magic: Sharran who, if touching light-caster with obsidian wand, absorbs and traps the spell inside the wand (but breaking or disarming the wand frees the spell in a radiant burst)
There's always something you can do to kill a PC!
