Sword of Kas

The  is a sentient, versatile black longsword once wielded by Kas the Bloody-Handed when he rebelled against Vecna. After Kas' death, the went missing in the depths of Thar Amphala until it was found by Keyleth. It was then given to Grog Strongjaw.

The seeks blood and the downfall of Vecna. As an NPC, the is portrayed by Matthew Mercer.

Appearance
The Sword of Kas is a longsword, and its blade is black, beautiful, and hooked in places.

Personality
The is singularly focused on destroying Vecna and his followers. Besides its thirst for blood, the main thing the Sword talks about is that "the lich must die." When used to attack Vecna, the Sword discussed wanting to usurp Vecna's power for itself.

Background
Vecna forged a relic blade for his new vampire lieutenant with a fraction of Kas's consciousness. Kas used it to terrorize Vecna's enemies for months. When Vecna first attempted to ascend to godhood with the Ritual of Seeding, Yos Varda attacked Vecna directly. Though Vecna defeated Yos, the effort left him winded, and as Yos lay dying, Kas picked that moment to attack Vecna with the blade in an attempt to usurp him. The ensuing battle destroyed both Vecna and Kas, but the sword remained—though it was not recovered.

Beaneath Thar Amphala, Vex used a Scrying Eye on the Sword and saw it submerged in muck, but got a good enough description of the object to allow her to cast Locate Object and discover that it was within 1,000 feet. They made their way directly above the object. Keyleth, in earth elemental form, earthglided down to a small chamber 150 feet below—a room cut off by a collapse—with a pool of acid that had been used for disposing of bodies and other unwanted things. Her elemental form incurred acid damage and avoided being grappled by undead skeletal hands to recover the Sword, and Vex (who had used Oil of Etherealness to drop down to the chamber, unaware that it would make her useless for a full hour) returned to the party with the Sword via a gate stone.

However, the plan to retrieve the Sword had some holes, and it revealed Vox Machina's location to Vecna. Under attack by the Briarwoods, they were forced to retreat to the Feywild. Grog immediately began attuning to the Sword, and the party was introduced to the Sword's motivations and its thirst for blood.

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 becomes gaunt, their skin turns 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.

Quotations

 * "The lich must die."
 * "Just wield me. Spill blood for me. Bring me to the Undying King and cut his throat with my blade. That's all I ask. That's all I need."