To acquire it, you need to already have another Feat related to it, for example : No prerequisites, anyone can take this Prowess.
Your Game Master may accept to waive such prerequisites if your background or in game actions justify it. Unlike normal Feats, a Prowess can be acquired several times.
You own and know how to operate an Arbalest, a fearsome ranged weapon.
Arbalests are forbidden in all Christendom and can only be used against pagans or heretics.
An arbalest takes 30 seconds to reload at best and will shoot bolts with formidable strength, enough to break through any armours and can even break doors.
Since this is a forbidden weapon, you have to conceal it at all times and cannot be seen shooting it at anyone but heretics or pagans, which is why you can only take this risk once per scenario.
If used against a target, this feat works like the True Strike (ranged) prowess, except for the fact that you need 30 seconds to reload it, unless you specify that you have reloaded it previously. This weapon is effective as far as 100m.
On a 5 or a 6 on a die 6, this Feat can defeat an opponent that would require the Combat (monster) Feat.
You can't leave it reloaded for more than couple of minutes, because that would damage it.
If used against any small object, like a chest or a normal door, it will very likely explode to splinters.