Warblade of Rak'Thun

Martial weapon (Greatsword), melee weapon, artifact (requires attunement)

5 lb. 1d12 Slashing

The bright crimson crystal blade made of crystalized blood is covered with swirling dark quasi material shadowmatter that crawls to your hand while you grasp it. You can rarely feel a single heartbeat from the massive crystal chunk while you are holding it.

Blade Bond. While you are attuned to the weapon, you know it's location at all times as long as you are on the same plane of existence. Additionally, you can’t be disarmed of the weapon unless you are incapacitated.

Blessing of the Ascended. If you are a worshiper of Rak'Thun, you gain the piety benefits shown below for which you have the required piety.

If you aren't a worshiper of Rak'Thun, the greatsword has the following detrimental properties.

While attuned to the artifact, you are physically ill and have disadvantage on any ability check or saving throw that uses Strength or Constitution.
While attuned to the artifact, you are blinded when you are more than 10 feet away from it.

Dormant

The greatsword grants the following benefits in its dormant state:

You gain a +1 bonus to attack and damage rolls made with this magic weapon, and it deals an extra 1d6 necrotic damage to any target it hits.
While attuned to the greatsword, you have resistance to necrotic damage.
As a reaction when you are hit by a melee attack, you can make a melee attack with Warblade of Rak'Thun with advantage against the attacker. You can't use this property again until you finish a short or long rest.

Piety 10+. While attuned to the artifact, you are immune to disease.

Awakened.

When the greatsword reaches an awakened state, it gains the following properties:

The weapon's bonus to attack and damage rolls increases to +2, and the extra necrotic damage dealt by the weapon increases to 2d6.
When you reduce a creature to 0 hit points with an attack using Warblade of Rak'Thun, you can cause the following effect: each creature of your choice within 15 feet of you must succeed on a DC 15 Wisdom saving throw or be frightened of you until the end of your next turn. This property can't be used again until the next dawn.

Piety 25+. While attuned to the artifact, your walking speed increases by 10 feet.

Exalted

The weapon's bonus to attack and damage rolls increases to +3, and the extra necrotic damage dealt by the weapon increases to 3d6.
The saving throw DC for creatures to avoid being frightened by it increases to 17.
When you reduce a creature to 0 hit points with an attack from the greatsword, you can regain hit points equal to the damage you dealt with the attack. This property can't be used again until the next dawn.

Piety 50+. While attuned to the artifact, you regain 1d6 hit points at the start of your turn if you have at least 1 hit point.

Origin of the Warblade

During Rak'Thun's fight against the lich Vecna and his right hand man Kas in the Shadowfell, he was struck by the Blade of Kas as his blood was spilled to the desecrated ground. To even the odds against the two entites, Rak'Thun's shadow rose from the ground and lifted the blood as a sword as it crystalized in its grasp. It is told that the blade was shattered when it was struck countless times by the Blade of Kas while the shadow was trying to hold him off, but not before Rak'Thun has dealt the decisive blow against Vecna, delaying his plans once more.

Destroying the Greatsword. To destroy the greatsword, it must be struck with the Blade of Kas in the Shadowfell by someone who is attuned to the Hand of Vecna and holding the sword with it.