Sword of Kas

The  is a sentient, versatile black longsword once wielded by Kas when he rebelled against Vecna. After Kas' death, the went missing in the depths of Thar Amphala until it was found by Keyleth. The seeks blood and the downfall of Vecna. As an NPC, the is portrayed by Matthew Mercer.

Features

 * Scores a critical hit on a 19 or 20.
 * Deals an extra 2d10 slashing damage to undead.
 * If the Sword of Kas isn't bathed in blood within a minute of being drawn from its scabbard, its wielder must make a DC 15 Charisma save. On a success, they suffer 3d6 psychic damage. On a failure, they are dominated by the sword (as if by dominate monster) and the sword demands that it be fed blood. The effect ends when the sword is fed.
 * Absorbs blood.
 * Random Properties
 * Minor Beneficial Property: Wielder gains proficiency in one skill while attuned.
 * Major Beneficial Property: Wielder's Constitution score increases by 2 while attuned, to a maximum of 24.
 * Minor Detrimental Property: Wielder's skin becomes pale and their eyes turn a faint red.
 * Major Detrimental Property: On attuning, wielder takes (8d10?) psychic damage.
 * Spirit of Kas
 * While the sword is on wielder, they add a d10 to initiative at the start of every combat.
 * The wielder can transfer some or all of the sword's attack bonus to their AC when they use an action to attack. This lasts until the beginning of their next turn.
 * Spells: While the sword is on wielder, they can use an action to cast call lightning, finger of death, or divine word from it (save DC 18) once per day. Each spell recharges at dawn.