Why implement this: Nidbane balancing is difficult because either they kill the criminal or don't. If they're strong the game is PvE, if they're weak they're just laughable. There needs to be in-between for balancing.
Overview of the idea: Nidbanes should be visible only to the criminal, and possibly summoner. Nidbanes should KO a criminal, debuff them, hurt their HHP a bit and then attack again in 24 hours. The wounds and debuffs grow more severe, and the X time the nidbane ko'd the criminal, it's a permakill. There are ways to make it easier to fight the nidbanes.
More details:
Visibility to only the criminal: Nidbanes seem they should be more of an illusion or going mad with guilt to the criminal. Outside assistance allowance encourages 1 UA criminal alts with "oh gotta wake up and clear the spooks with my fighter" metagame-ry. Instead, outside assistance should come through other things (putting a section on this)
KOing, debuffing, wounds: The idea is that if you can't beat the nidbanes the first time, you're not going to the second time. The debuffing combat stats and HHP damage makes you weaker in the real game, and with your mental spooks. Instead you need to get help.
Spook Battles Part 1: The first way to improve your chances against the nidbanes is to appease the scent you made. This was kind of done with the death patch where burying the victim weakened the murder scent. A way this could be done for example: a nidbane was summoned for a theft scent of a 50/50/50 bronze sword, and the criminal can return that sword, or one of equal/higher quality to weaken the nidbane or banish it. Similarly, a vandal nidbane for a 30/30/30 kiln can be offered clay of equal or higher quality.
Spook Battles Part 2: Electric Boogaloo. The basic idea is implementing some form of random offering, which will strengthen your ability to fight nidbanes for a period of time, or weaken a particular nidbane. The offerings could go back to the nidbane's altar or just disappear. I'm thinking something akin to ancestral worship where you have to build up to a certain amount of "numen" and then can activate the prayer.
I won't go too much into the specific details and logistics for nidbane weakening/power levels as jorb usually hates ideas that are in-depth but aren't his. This is just intended as sort of lever to allow balancing of nidbanes.
There will probably need to be lots of explanation text to not confuse players when they see the nidbanes.