Augment Obsidian

Spell Lists: Druid, Ranger, Sorcerer, Warlock
1st-level transmutation
Casting Time: 1 bonus action
Range: Touch
Components: S, M (a fist-sized chunk of obsidian worth at least 5gp which the spell consumes)
Duration: 10 minutes

You infuse a melee weapon you touch with an outer layer of brutally sharp, brittle obsidian shards. For the duration, the weapon requires at least 15 Strength to wield without disadvantage. Melee attacks with the weapon deal an additional 1d10 bludgeoning or piercing damage (your choice) on hit.

Each time the affected weapon hits with an attack, bits of the obsidian layer break away, reducing the extra damage added by the spell to subsequent attacks. On a critical miss where the attack roll is a 1 on the d20, the obsidian layer breaks away twice. No obsidian breaks away if the weapon scores a critical hit, or if the attack roll exceeds 25.

When the Obsidian layer breaks away, the damage is reduced from a d10 to a d8, then from a d8 to a d6, and finally from a d6 to a d4. When the spell adds only a d4 and the obsidian breaks away again, the spell ends.

At Higher Levels. When you cast this spell using a spell slot of 3rd level or higher, the attack total threshold for sustaining the obsidian layer is reduced by 5 for every two slots above 1st.