Using wisps of shadow material from the Shadowfell, you materialise Ark, the Dark Harbinger.

A Gargantuan manifestation of Ark appears for the duration, hovering 2m above the ground in an unoccupied space that you can see within range. Ark is considered to be a creature that has a damage threshhold of 20, and HP equal to 150 + half of your maximum HP. If he drops to 0 HP, the spell ends. He knows how to speak and read Ivalician and Infernal, and you can communicate telepathically to one another. He has the following stats:

When you cast this spell, and as a bonus action on your turn, provided that Ark is within 100m of you, Ark can move up to 20m (he is considered to have flying speed and can hover), and you can mentally command Ark to do one of the following abilities:

• Invisibility: Ark extends a field of invisibility which emanates 20m from him. While this effect is active, Ark, along with any creatures on board, are considered
  Invisible
  by those outside of this field. This field of invisibility ends when Ark performs another command.
• Shadow Ray: Ark makes a ranged spell attack (+8 bonus to attack roll) against a target within 100m. On a hit, the target takes 4d8 Dark damage.
• Force Field (recharge 5-6): Ark creates a 2cm thick, 20m × 20m shimmering translucent magickal wall of force lined with glyphs directly in front of him, which moves with him and remains until the start of your next turn. Nothing can physically pass through the wall, and it is immune to all damage. The wall also extends into the Ethereal Plane, blocking ethereal travel through the wall.
• Eternal Darkness (once per casting): Ark uses his ultimate move. All hostile creatures within 50m of Ark must make a DC 18 Constitution saving throw as a wave of negative energy emanates from him. This energy pierces through most cover, but is stopped by 60cm of rock, 5cm of metal, or a thin sheet of lead. On a failed save, they become
  Blinded
  and
  Paralysed
  for 1 minute. A creature can make a Constitution saving throw at the end of its turns, and ends one of these conditions on a successful save.

When this spell ends, Ark persists for 1 minute before fading into the shadows. During this time, he cannot be commanded by you and slowly descends at a rate of 20m per round.

At the DM's discretion, Ark could perform other actions, such as interacting with objects. Ark is a Neutral creature, so if you or your allies are performing acts strongly not aligned with his, Ark can force you to make a DC 15 CHA save at the end of your turn. On a failed save, Ark can dismiss himself back to the Shadowfell. On a successful save, Ark remains in your services, but the DC of this CHA save increases by 5.