From some thoughts I had while reading Monster Manual 2 and thinking about survival horror. Feels a bit of a work in progress and I wish I could find a suitable pic.
By stun, confusion, save, advantage and damage etc, I don't necessarily mean what's in any particular rulebook, but they should be familiar/generic enough terms for this to be system agnostic.
Decide if it's unique or a monster species. Prevention and/or cure of special effects as for your system and setting, or a blessing from the right priest.
- Armour Class: as leather, scale or plate (your choice)
- Move: 9 - it can climb walls, cling to ceilings, move through treetops and across webs, but usually just squats in place
- Hit Dice: 5 (but it's got damage resistance, too)
- Attacks: 2 (which you can treat as whips) + special
- Intelligence: it can have Unratable with Neutral (evil) Alignment, which seems to be the default for alien intelligences (at least in AD&D MM2) - it's probably immune to charm.
When it shifts position - which it does so only seldom and slightly - a hint of its hidden form might peek out from beneath the debris - glistening chitinous claw, slender antennae, wan submarine glow of featureless orb, hairy tips of dozens of bony feet. The tarp is not a tarp but a skirt of tough fungus.
The Ghost of a Death Watch Beetle can only be harmed by magic, silver weapons and fire. It takes only 1 hit plus any magic bonus from weapons, and only 1 hit per damage die from fire because it is so wet and slimy (it does not always appear so due to a clinging layer of dust, debris and powdery spores).
It cannot see but can sense all movement around it (to 120', if you're measuring), and when it does it will start to click quietly - so quietly you'll need to roll Hear Noise to detect it with only 10% chance of pinpointing the source.
After 2 rounds, save or suffer Fatigue. It's audible as background noise, like the ticking of a clock or a Geiger counter.
After 4 rounds, save or suffer confusion. By now you can hear it all around you, nearly impossible to pinpoint the source (only 10%, and a critical fumble means you blunder straight into it's clutches, automatically surprised).
After 6 rounds, it's booming and cracking and tolling like a catastrophe - Morale and Loyalty Checks for everyone who has to make them. No communication is possible - even mind-to-mind requires both parties to do nothing else but concentrate on doing so. Spell casting is at disadvantage, however you rule that (chance of failure, mishap, corruption, cost in energy is increased etc).
The clicking abruptly ceases on the 7th round and everyone still in range must save or be inflicted with the Quivering Palm - in 5 days, anyone who succumbs will suddenly and gruesomely vibrate to death. Up until then, they will seem to have a slightly fuzzy outline, a metallic timbre to their voice and mild delusional parasitosis - they don't feel the cold as much, either (advantage to saves, reduction to damage).
The Ghost of a Death Watch Beetle is stunned by exposure to light, delaying or interrupting its clicking for 1-6 rounds depending on the brightness. Being exposed to daylight prevents it clicking at all until it has retreated into the darkness to recover.
Anything that gets close enough will be attacked with two long and slender fishing rod-like antennae that shoot from beneath (or through holes in) the tarp. These drain 2 Dexterity per hit by otherworldly chill (or use an alternative - even classic Energy Drain), and on a natural 20 whip around the throat of a victim and strangle for automatic damage - use your preferred mechanic for this (there are plenty). Treat the antennae as actual whips if you prefer.
Living beings reduced to 0 Dexterity (or slain) are released, having become the shambling servants of the Ghost of a Death Watch Beetle. They are will-less zombies and can only attack by strangling (again, use a preferred method). Servitors also produce a clicking noise not unlike that of the Ghost itself, though only causing confusion at most and over turns, not rounds - penalise saves for multiple clickers. Multiple clicking servitors makes it even harder to locate the Ghost.
All Ghost servitors will eventually die of natural causes, but will survive as some kind of fungoid undead until they rot to uselessness or are completely colonised. Fungoid monsters will associate with the Ghost, and there is communication between them.
The Ghost of a Death Watch Beetle is neither undead, nor arthropod, nor plant - but speak with dead and speak with plants allow communication, though there is still the gulf of incomprehension between minds so alien to each other. It can sometimes speak through its servitors.
If slain, it collapses into a pile of fungus-infested rubbish and liquefying slime, but will reform in a week if not exorcised - it can also be destroyed for good by spells such as dismissal, plane shift and dispel evil.
The Ghost of a Death Watch Beetle and its servitors are unable to cross a protection from evil circle.