An obsidian sword, finely balanced. Its edge remains sharp even without maintenance. The blade is tinged green, ever so slightly.

You gain a +1 bonus to attack and damage rolls made with this magic weapon. On a hit with a natural 18 or higher, the target takes an additional 1d4 acid damage.

The sword is, in fact, alive. It once was a common Mimic up to its usual masquerade, hoping to devour a greedy adventurer. Fate, however, had other plans. A powerful spell petrified the Mimic and kept it from transforming. Now, eras later, this sword-mimic falls into new hands. It carves through enemies against its own will, building up rage towards its unsuspecting wielder. Perhaps enough stress might break its ancient magical binding. 

On a critical hit or a natural 1 with the sword, the mimic has a chance of breaking free from its spell. If this occurs, it will spring out of its wielder's hands and furiously attack them. Livid at its humiliation, it will hunt down its former wielder relentlessly and exclusively.

The initial chance of this happening is 1%, but with use and abuse, this will creep higher.
Each critical hit raises this chance by 1%. Each natural 1 raises this chance by 2%. Lighting the sword on fire or any attack specifically targeting the sword that hits it raises this chance by 1%.
If the sword is targeted by the Dispel Magic, Remove Curse, or Greater Restoration spells, or enters an Antimagic Field, it breaks free instantly.

If targeted by the Identify spell, the caster learns only that the sword was affected by the Flesh to Stone spell.

Proficiency with a longsword allows you to add your proficiency bonus to the attack roll for any attack you make with it.

Notes: Damage: Acid, Bonus: Magic, Versatile