Vhagar's Shadow

Vhagar's Shadow is a highly recognizable sorcery involving the manipulation of one's own shadow adapting concepts from various sources. It's very rare to see because it relies on Cotd eyes to manipulate while having a non-negligible sustained cost, making it impractical for them to use due to their tendency to be Empty. On top of that, it requires an extreme level of Celestial knowledge and the ability to mentally execute encoding algorithms that would make archmages cry. Realistically, it only makes sense for Vhagar to use it: the only other Shrouded for whom it would be within reach is literally composed of light, making her incapable of casting shadows.
In terms of practical effects, it exists in two parts: the ability to extend a claim through nearby shadows, and access to a 'pocket dimension' through those shadows. The mechanism behind the pocket dimension is a mix of the ghoul flesh-well, limited shift illusion, and soulscape manipulation. It draws in concepts similar to the northern shadow binding ritual, though it was created long before arcanism's spread. It's not a true pocket dimension so much as a blurring of boundaries between Celestial and physical that allows one to mimic the ghoul flesh-well effect across shadows instead of a fixed point (the ghoul's heart).
The casting medium exhibits qualities of illusion and shadowlink, though the structure is different from both. It's a more refined version of what infernal nightmares do, and uses similar mechanisms to the nightmare reality override.
An odd quirk of the magic is that unlike ordinary darkness, it doesn't generate a minor ambient light component (while mundane umbrite is as 'bright' in darklight as hard light projections are on the visible spectrum).
Limitations:

• Upkeep cost scales with amount of shadow claimed.

• Only things under one's own Claim can enter the pocket dimension.

• Objects entering the pocket dimension have to be 'encoded' into Celestial. This is the real killer. It's the main reason why Murmur just sticks to claiming water.