Unfathomable! The Psychephage Scenario (Statblock and Tactics)

Our most recent session began with the PCs facing a horrible, brain sucking monster, the Psychephage. It was a tricky scenario to set up for the following reasons:

• There were 7 PCs and 3 NPCs versus 1 creature, so they could easily wipe out even a very tough monster before it ever took a turn.
• The PCs are only second level, so if I beef up the monster too much, it could end up one-shotting a bunch of them.
• The PCs have some very strong magic items, although they tend to forget about them. If used creatively, these could produce all sorts of unforeseen battle conditions.

I'd started out with a fairly standard statblock for the Psychephage. I think it was about a CR3? or maybe 5? I don't tend to pay a lot of attention to CR. This is how I adapted it to try and give my players a sustained challenge:

Psychephage 

A debased strain of Blind Antler Man that has devolved into a specialized intellect eater. 

Medium Creature

AC 14    HP 100    Speed 30'

STR +2    DEX +1    CON +5    INT -2    WIS +4    CHA -1

Saving Throw Con +4

Skills Athletics +4

Damage Resistance to mundane weapons

Senses blindsense 30', intellect detection 80', passive Perception 14 

ACTIONS

Multiattack. The psychephage can make 2 claw attacks or 1 claw and 1 proboscis attack.

Claw. +5 to hit,  9 (1d12 + 3) damage. DC 14 STR or DEX Save or be grappled.

Proboscis. Target must be grappled. CON 13 Save or expend 1 Hit Die. When all HD are spent, the target drops to 0 hit points as the psychephage consumes their intellect. Consuming a creature's intellect heals the Psychephage 1d10 x the Target's Intelligence Score. If it consumes a brain containing memorized spells, these are instantly and simultaneously cast, targeting anything nearby at random.

Legendary Actions

The Psychephage can take 2 legendary actions, choosing from the options below. Only one legendary action can be used at a time and only at the end of another creature’s turn. The psychephage regains spent legendary actions at the start of its turn.

Guise. The psychephage projects an illusion over its body, appearing as a creature whose intellect it has previously consumed. It can imitate the creature's voice, but any speech is meaningless gibberish.  DC 13 Intelligence (Investigation) check sees through this illusion.

At the beginning of combat, the Psychephage's Guise repertoire consists of a single Ancient Beetle scientist.

Hallucinatory Terrain. The psychephage projects an illusion over the surrounding terrain (30' radius) to conceal itself from prey and to change the positions of any targets relative to one another. DC 13 Intelligence (Investigation) check sees through this illusion.

Mass Guise. The Psychephage projects the illusion of its own appearance over creatures in a 30' area. INT 13 Save to avoid taking on the illusory appearance. Any speech or actions taken by an target of this ability will be obscured or reinterpreted by the illusion. Taking an action to perform a DC 13 Intelligence (Investigation) check sees through an individual instance of this illusion.

Design Notes

I boosted its HP and gave it resistance to mundane weapons, but kept its AC low. I wanted it to last long enough to activate the hallucinatory hijinks, but I also wanted the PCs to be able to land blows when they found it among the illusions.

I don't know how often GMs use Legendary Actions against 2nd-Level PCs, but it seemed the logical way to balance the monster's action economy against all those PC actions.

Lots of hallucination powers are intended to keep the players engaged in something other than just wailing on a shared target.

I then planned out a theoretical order of actions, just to make sure I was thinking through all the details.

Psychephage Encounter Order of Actions

1. Roll PC Initiative 

2. Round 1, Initiative 18: Psychephage multiattack's a PC

3. After next PC action: Legendary Action 1: Psychephage casts Hallucinatory Terrain

1. Mist-shrouded landscape with undulating shag-terrain, alien tumor-plants, and a lava-lamp sky

4. After next PC action: Legendary Action 2: Psychephage casts Mass Guise

1. Ask PCs to roll INT checks in private and pass them to the GM (DC 13)

2. On Failure: Illusory transformation into a psychephage. On Success: no illusion.

3. Everyone's positions are switched around.

4. Attacking any "psychephage": roll to determine who is hit.

5. Round 2, Initiative 18: Psychephage multiattack's a PC

6. After next PC action: Psychephage casts Hallucinatory Terrain again, changing everyone's positions relative to one another.

7. After next PC Action, it will use Guise to appear as a Beetle Scientist

8. After next attack directed at Psychephage: it figures out that Beetle is not a good disguise and drops Guise 

9. At this point, the Psychephage will alternate using Hallucinatory Terrain to hide itself and multiattack ambushes. It begins starved, and will fight to the death until it has consumed one intellect. After that, it will flee if it feels overwhelmed. It may choose to stalk the party and attack during a long rest.

Maps

I made a quick map for the initial terrain:

And then googled "Lisa Frank AI Wallpaper" for appropriately eye-seering Hallucinatory Terrain:

All in All

The scenario worked satisfactorily at the table. Full session report forthcoming!