This item appears to be a flowing, black-silver mass that typically takes on the form of a staff. The item has the following features:
Transformation. As a bonus action, the wielder can speak a command word that they create, and can choose what weapon this mass takes, excluding any staff, or arcane focus of any kind.
Adaptability. When this item takes the form of a weapon, it takes all of its stats (damage die, weapon type, etc.).
Freedom of Will. Once Transformation is used, you can speak a command word and have the weapon fight on its own. This weapon is now considered to be another member of the party and continues fighting until the user drops unconscious or ends the effect as a bonus action. Once this entity has been "animated", it rolls its own initiative, has a flying/floating speed of 30 feet, and will target the nearest enemy creature on its turn. As a bonus action, the weapon may take the Dash action. The weapon makes two attacks when it takes the Attack action and has a +9 bonus to hit.
Proficiency with a Spear allows you to add your proficiency bonus to the attack roll for any attack you make with it.
This weapon has the following mastery property. To use this property, you must have a feature that lets you use it.
Sap. If you hit a creature with this weapon, that creature has Disadvantage on its next attack roll before the start of your next turn.
Notes: Damage: Slashing, (requires attunement), Thrown, Versatile, Sap