Umbra's Aid

Upon casting this spell and touching a willing creature, you may reduce the creature's maximum HP by half through manipulating their soul. (Round down) The lost HP is then merged within the creature's own shadow, giving the creature an Umbra.

The Umbra is the creature's shadow personified. It is entirely loyal to it's creature and can be mentally controlled by the creature. The Umbra has identical attributes to its creature other than it is entirely black and weighs nothing.

The Umbra, unless commanded otherwise, stays attached to it's creature as a regular shadow. While attached in this way, it takes the help action as a reaction to whenever the creature makes an attack roll, ability check, or saving throw.

While detached the Umbra takes it's turn after it's creature, and can take any action it's creature can, but is unable to deal damage to other creatures. If the creature takes an action with limited use, such as a spell or class feature, it uses it's creature's limited uses for that action.

When the Umbra reaches 0 HP or this spell is casted again upon the Umbra, the Umbra is destroyed and it's creature has it's maximum HP restored, but not healed.