Working my way through Vornheim. Some clever ideas here for running a city campaign. But what spiked my curiosity were the unique takes on some monsters and some of the unique monsters. Can definitely imagine using these in my 5e OSR game.
This would probably work well as a template as well. Based on the dividing demon with some inspiration from Sticky Fingers.
Human sized, bent, gaunt and gibbering, leathery black skin, red eyes.
Mythical Magic Resistance. Any spell cast by a level 8 or lower spellcaster automatically fails when targeting the zipper demon, otherwise, the zipper demon has advantage on saving throws against spells and other magical effects.
Reactive. The zipper demon can take one reaction on every turn in a combat.
Sticky Fingers. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 1d6 + 2 piercing damage and if the target is Medium must make a DC 16 Constitution saving throw or be divided into two halves. These two halves can not be reconnected until the demon is slain or banished for the same plane of existence. Each half has all the abilities and traits of the original creature but half the hit points. The exposed half of the divided creature looks like a purple-hued milky way.
Severance. The zipper demon chooses to divide itself into two halves by unzipping itself and splitting apart. These two halves have all the traits of the original zipper demon but half the hit points and only one sticky finger attack. This action can also trigger the meld reaction.
Meld. If a Medium creature within reach is divided the zipper demon may make an opposed Strength (Athletics) check to zip itself to the target. While melded with its target in this way the zipper demon has access to all the targets abilities and traits as well as retaining one zipper claw attack.
Division. When a whole zipper demon is subjected to slashing damage it may use the severance action.
Note: This demon can only inhabit rooms with an even number of living creatures in them (including the demon itself, but not including plants, microbes, insects etc). Otherwise, it will flee to the nearest room which fits its requirements. (If the demon slays a living creature it will usually have to move.) The demon cannot go outside and cannot be alone. If it is unable to find any room it can occupy it will scramble, panicked, from room to room until the situation changes.