Venom Strike

A dark purple fang sprouts around your melee weapon. As part of casting this spell, you must attack with the weapon used as the component of the spell. If the attack hits, on the start of your targets turn they take 1d10 poison damage.

The spell increases in damage at certain levels. At 5th-level your initial hit deals an additional 1d8 poison damage and 2d10 at the start if their turn.  At 11th level 2d8 and 3d10 at the start of their turn. At 17th level 3d8 and 4d10 at the start of their turn.