The Soultaker Sword

Consider this a teaser of what you will see during the A to Z Blogging Challenge.

Wielded by the Outsiders and Suicide Squad's Kitana, the Soultaker is a magical Katana capable of absorbing the souls of those it kills. The bearer can communicate with the souls and use their knowledge. 

Soultaker 

Weapon (longsword), legendary (requires attunement)

This magical katana grants its wield a +1 to hit and damage with the blade. Any creature killed by the Soul Taker is absorbed into the blade. They may not be reincarnated, resurrected, or be turned into intelligent undead by any means (with the possible exception of a Wish spell at the DM's discretion). They may call upon the knowledge of the souls trapped in the blade. As an action they can use this knowledge to gain proficiency in all skills of a chosen Ability. The duration of this feature lasts until a Long Rest. Once this ability is used it may not be used again until the wielder takes a Long Rest.

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If you've played Baldur's Gate 3 the swords ability to draw upon the knowledge of the souls trapped within will look familiar. It’s the same as the Githyanki's Astral Knowledge ability.