Craven Edge

 is a large, thick, onyx, two-handed greatsword once wielded by Sylas Briarwood and then by Grog before being thrown into a pocket dimension. It appears to have a presence of its own, with sentience and intent. Imbued with dark arcane power, it can drain strength from the foes it strikes.

Matthew Mercer released the full description for Craven Edge on Dropbox after.

Telepathic Communication
"You... are you my new wielder? [...] Leave me until one worthy lifts this blade. I am useless left here; I seek the thrill of battle—I am hungry."

"Do you enjoy drinking the strength of your foes, watching them beaten before you, and have that very power become your own?"

"Do you wish the shadows to encase your form and grant you resilience in anger, forcing those to kneel before you in fear?"

"No... Keep the tears flowing."

Bonuses

 * Grants the wielder advantage on Intimidation (Charisma) checks when unsheathed.
 * Saps the target's Strength when struck by the blade, draining a point that goes to the wielder till the next short rest. The limit to this ability is 25 Strength (+7 modifier). On a successful save, this effect is ignored.
 * When Craven Edge hits its limit of drained strength, it extends and becomes a more gruesome-looking blade. Once extended, the blade does an additional 2d6 Necrotic Damage upon hitting a foe.
 * The next rest after reaching its extended state, the wielder must make a Constitution Saving Throw or have his or her soul taken by the sword.

Trivia

 * Craven Edge is an anagram for "Vecna Greed".