Yeah it would. Long dagger + Buckler with high ath and shield skill. 1 hit people in heavy armour who cant get away even though they have 18+ str.
This idea is plain stupid.
It wouldnt be OP if played how you suggest but your ignoring its potential.
Already suggested 'canot be used with shield' and potentially 'cannot be used with 2-slot weapons'
Also, as i previously said, it would need some 'charging time' or something, so it takes some time to do it making it not viable in 1 on 1 combat, but
only if you get behind an unaware enemy.
Well, if you give it the 'can't be used with a shield' tag it could work. Also, if it would happen, it should get unsheathable tag or get a lot more expensive, since guaranteed one-hit kills are such a big bonus.
Unsheathable would be weird imo, a price increase would seem reasonable - perhaps even add a new weap with this feature so people still get to use the cheap dagger. Most people who would se this, would be in lgiht armor anyways, and they would have loads of money. 5k would seem fair imo.