Black Velvet Inferno

In a whisper, no louder than a butterfly, you summon a mass of opaque energy.
Moving like a swarm of black butterflies, softly and quickly, deadly and swiftly.
It will entrap your targets, in a 6 ft/5 m wide sphere, it is unbreakable.
The round walls are made of necrotic energy, striking those will counter with 1d6.

When the round prison is entirely hermetic, we can't hear nor see what's inside.
The weight of it makes it damaging the ground it is standing on, slightly sinking.
Gleaming cracks will appear and grow wider as they start from the top to the bottom.
The walls will slightly shrink, to finally burst from the centre, a blast of searing white shockwave.
As they are trapped, 6d12 necrotic damage with advantage.

As the deed is done, the cracks now separated will fall on the ground and vanish.
Everything in the radius of the sphere is now atomized, only left is a round carved and cold ground.