Death Blade of Eiynar (Patreon)

Any sword, legendary (requires attunement)

Half the blade of this sword is consumed by a dark material that flickers with white energy. You gain a +3 bonus to attack and damage rolls made with this magic weapon. Every time you reduce a creature to 0 hit points with this weapon you capture a fragment of its soul within it.

Castle of the Dead. As long as there is at least one soul within the sword, you can use your action to summon a portal to the Castle of the Dead. The portal is a 5-ft-radius dome of force that floats 5ft above the ground and remains stationary for the duration. Only you and up to nine willing creatures of your choosing can pass through the portal. 

The portal takes you to a Demi-Plane where all the souls you trap within the sword are forced to obey and serve your commands. The Demi-Plane is an empty castle, with bedrooms, an armory, a kitchen, a library, and a great hall. You can stay at the castle for up to 8 hours, after that the portal takes you back to the place you were. You can't use this effect again until the next dusk.

Calls for Redemption. As an action, you can speak the sword command word and summon the trapped souls within to your aid. You can summon up to 5 souls at the same time. The souls use the Ghoul stat block. You can use a bonus action to mentally command the souls, issuing the same command to all of them. 

To receive a command a creature must be within 60ft of you. You decide what action the creatures take and where they move during their next turn, or you can issue a general command, such as to attack someone. Once given an order, the creatures continue to follow it until the task is complete or until it's destroyed. 

At the end of 1 hour, or if you decide to end it early, the Soul dissipates into the air, free of the sword's grasp. You lose a number of Soul Fragments equal to the number of souls you summoned. Once you use this effect you can't use it again until the next dusk.