Ancient Borders (UK TV Documentary c1993)

I don't know whether this is fiction, non-fiction or if that even matters.

Maybe this is some riff on A Connecticut Yankee in King Arthur's Court or maybe not?

My trusty Brewer's Dictionary tells me that a Mercy Dagger was a small scalpel-like knife used to deliver a fatal blow to a fallen, injured foe

Geervos Curse 

I'd say it was a mercy if your choices were this or crows chewing on your not quite dead flesh after falling in battle