Spellrender

Weapon (longsword), uncommon

This magic weapon has the finesse property. When you hit a creature with the weapon, you can choose to hit it with the inverted, serrated side of the blade: the damage from the attack is halved, and the target suffers a magical gash until the start of your next turn. The first time a creature begins to cast a spell while affected by the gash, the wound flares and deals 2d8 force damage to the target. Once you apply a gash, this property of the weapon can't be used again until the start of your next turn or until the target casts a spell.

"Oh this?" The man looked down at the weapon in his hand with a sneer. "Took the handle off the hands of an old mage. Vile thing, them, ye know?"
A young man stood before the brigand. He wore the tell-tale garb of a wizard's apprentice. "Some, yes. I, too, am a mage, and don't think myself vile."
"Well that's the trick of it all then, ain't it? Villains ne'er think themselves the bad guys 'til someone like me comes 'n' tells 'em. Often with a weapon like this."
The man brandished the curved blade in front of him, cocky in the presence of his goons on either side.
The young wizard continued, unfazed, "And am I to understand that this is you attempting to alert me to the fact that I'm a villain?"
The sneer widened. "Eh, ye catch on quick now, don't ya? C'mon then, it's three to one, bes—"
A whoosh of fire roared past the man's left and right sides, scalding him and incinerating his lackeys.
"Let's count again."
....
Moments later, the young wizard kneeled down to peel the now-hot blade from the highwayman's charred hands.
"I'm sorry your staff was repurposed to such ill-fitting ends, master," he muttered as the darkened fingers crumbled away. "I'll put the blade to better use. The evil amongst us needn't use our craft for wretched gains."