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on table they blow away most regular troops i was wondering how powerful are they in the fluff?
any examples?

It's a demolisher cannon...
Anything not in terminator armour is gonna be vaporised and even then they might not make it.
Vehicles smaller than baneblades are gonna be destroyed by a direct hit though some of the tougher ones might be able to make it with some damage.
Basically it is designed to break open fortifications at point blank range, infantry and vehicles don't have nearly the same kind of defence.

Not really many examples off the top of my head, but the idea is that like a demolisher, it's a siege tank.

It's job is to target spots in a fortress' wall and make it not be there any more. I always imagined it turning towards a group of infantry, firing, and said infantry just being replaced with a cloud of dust and pink mist with a loud krump.

Most people can't stand up to a siege weapon and live.