|The Trapped Knocker|
The Trapped Knocker
It's a small threat, an annoyance. It deals 1d4 damage, 1d6 for the bigger instances. One can literally not see where the hit comes from. Adventurers that know of the beast understand that it can only hit within 10 or 20 feet from its "contrivance".
Someone constrained a soul in a contrivance and the poor thing's only way to communicate is to deliver invisible punches. It is seen as an effective way to make a worshipping site a bit more supernatural, and secure.
There are three ways to defeat a trapped knocker (or whatever name your adventurers devise for it).
Firstly, it's to avoid it altogether. In some sites, there are oral or written instructions revealing the presence of this unfortunate guardian and indicating the safe way around it. When such instructions are given, it gets easier to locate the contrivance. Some knockers are intelligent enough to obey a warning sign, or a compensation offering (it delights in crushing it, whatever that is, ... a knocker requesting flowers as offering is hard to satisfy in winter).
Secondly, if the contrivance is found, it can be destroyed. The magical force in the contrivance and the contrivance itself have to be destroyed. Each round of attempt grants one punch to the knocker, a reserve of compensation offerings is a must.
Thirdly, the contrivance can be unpacked, releasing the soul. It has to be done carefully, in a definite sequence. Instructions are necessary. Each round grants one punch to the knocker.
The evil person that packaged the contrivance had to actually kill to procure a soul. The level of intelligence of the victim creature determines a lot about the knocker. A rat soul might accept a piece of rotten biscuit as offering, as the scale of intelligence is climbed, behaviour varies. Cat based knockers are said to be quite vicious. Humans are rarely used, the cost being prohibitive.
Hiding the contrivance is as much an art as packaging the contrivance is. Some sophisticated architects place them in bricks or in the fold of convoluted fragments of stone, and cement the contrivance is the place to safeguard.
It is said that a single knocker downed a whole party once. It punched the first adventurer. That adventurer punched his closest companion, believing they were the offender. The fight generalized, peppered with knocker punches, and ended with all the party skulls crushed on the floor.
Those punches are cold, the first of them comes as a surprise, unseen. The main effect is mostly confusion.