Oswin's Longsword

The face of Bahamut is etched into the pommel.
Pommel Bash. When a creature within 5 feet of
you casts a spell, you can use your reaction to
make a melee weapon attack against it for [Str
modifier] bludgeoning damage. On a critical hit
the creature must succeed on a DC 14 Con save
or have its spell countered.
Damage: 1d8 (one-handed) or 1d10 (two-handed)
Damage Type: Slashing