Spellthrust Poinard

Weapon (dagger), rare (must be attuned by a spellcaster)

This delicate knife has a long, slender blade and grants the wielder a +1 bonus on attack and damage rolls. In addition, when making a melee spell attack, an attuned wielder gains a +1 bonus on their attack roll, and if the attack hits the DC of that spell is increased by 1, or by 2 if the attack is a critical hit.

In addition, on a critical hit the target takes a -1 penalty on saving throws against spells for 1 minute. If they fail a saving throw against a spell, the penalty disappears.