It's common knowledge that currently Nidbanes are useless, since they are so easy to dispose of. At the same time we don't want a situation where they are too powerful and a lackluster way to kill an enemy.
In my opinion large part of the issue comes from the fact that they can be fought, as PVE combat in haven can be easily cheesed. So I would propose the following:
1. Nidbanes cannot be fought
2. They will chase their target, slowly increasing their speed, until they deal set amount of damage, after which they disappear,
3. Damage would be dealt over time, only when nidbane touches the target, at a rate of 1 damage per minute.
4. Total damage would depend on the scent. For example theft / battery scent will make nidbane deal 30 damage in total. Vandalism: 50 Murder: 120 (numbers to be tweaked, should they scale with quality?)
5. Damage from nidbane would inflict a slowly healing wound, which would also debuff skills like UA / UC / stealth and stats like int / agi / str / con. The con debuff would only scale with wound size, not total con, which would make it easier to kill crime alts, to a point where a single theft nidbane could kill 10 con alt.
6. Current behaviour of Nidbanes reaching offline character would remain.
So every Nidbane would come with a guarantee of dealing some damage to the target, and with enough Nidbanes it might be possible to kill it. The target might be forced to run away and manage taking Nidbane damage over time so that they don't stack high enough to kill it. Although if the target left a lot of scents, and his enemies were dedicated Nidbane summoners then he wouldn't have much chance.