Sorry, couldn't resist all the alliteration. ;)
Reading the fourth and fifth Black Company books inspired this. Most likely to be encountered by my Wednesday group, but considering how ubiquitous spell-use is in 5e, it seems it would have very broad application.
So, somewhere in the mists of time, someone hated spell-slingers. Someone hated them A LOT. And they devised multiple methods of killing them. One of the most effective, and enduring, was weeping iron. Weeping iron looks like black iron except it weeps a nasty purple oil that coats the metal. Most weeping iron weapons are enchanted (because, apparently, they didn’t hate all spell-slingers, or maybe they were one flavor of spell-slingers with a hate-on for another flavor; whichever works best for your campaign, naturally).
Regardless of whatever enchantments a weeping iron weapon has, anyone struck by such a weapon must make a CON save (usually against 16) or be poisoned (as per the condition rules in the PHB). If the victim doesn’t have any spell slots available, they’ll shake the poison off in 10 minutes.
On the other hand, if the victim does have spell slots, they have disadvantage on the saving throw and immediately take 1d6 damage per the level of their highest remaining spell slot. (So if a wizard has two 1st level slots and a single 3rd level spell slot remaining, the wizard takes 3d6 damage from the poison.) Every hour after, the spell-slinger takes another d6 damage per level of their highest remaining spell slot. The poison feeds on the spell-slingers magical potential; only magic will serve to purge the poison from a spell-singer’s body.