Grief's Box

''“A cube item blossoms out of her. As light is radiated and freckles of stars cluster about, the item reshapes itself into a box.”''

Chise hands Grief's Box over to Fik in Ren'ai-Vega.

Although not directly stated, this is a box of literal Grief. It’s extracted by the Soul and the magic that entices. A personification of Fik’s demon coupled with her bloodline. It can transform into other ‘oddities’ when released.

Grief's Box (Story)
Alternatively, there is a short story sharing the same title as the magical item.

It centers on siblings Nade, Zuto, Ryuu, Ai and Sei. The story is implied to take place after the events of Oddity.

Grief's Box tackles a group of five siblings and their emotional journey with overcoming their mother's death; their grief being personated as a creature from the magical box.