Instead of killing the enemy, the wearer will drink the holy water (or whatever the hell this is) and will recieve the stats of the weapon
My stats idea:

Slayer's Sacrament:

Fully heals allies on weapon radius as well as removing Fire, Jarate and Mad Milk effects
Enemies will be marked for death for 15 seconds
Enemies will recive damage equivalent to what they do to your allies for 15 seconds
Enemy Pyros will die instantly (optional)
On self-hit: wearer will recieve invencibility for 10 seconds

On self-hit: wearer will be unable to attack for 15 seconds
Friendly Pyros will also die instantly (optional)

The sniper drinks the holy water, and winces, as if expecting something terrible to happen. Nothing does. He breathes a sigh of relief, but starts to sneeze on the exact frame the sneeze occurs, he explodes, killing himself and anyone nearby. Comes from the belief that sneezes are the devil in your throat, which in itself is the origin of saying bless you after someone sneezes.