Gather Cube

Gather Cube
A dark gray cube with a green center able to be seen from its 3 visible sides.
Type	Key Items
Effects	A Mysterious Cube that has the power to gather various types of cells.
Source	Ayuda in East Gearen Help Center
Cost to buy	n/a
Cost to sell	n/a

Gather Cube is a Key Item in Rejuvenation.

Effect

It stores Zygarde Cells, Zygarde Cores, and Red Essence the Player gathers around the region.

Zygarde Cells/Cores

A Zygarde Cell is a cell-like component of the Pokémon Zygarde that makes up the majority of its body, together with the Cores that command them. The cells can be found all over the region, while the cores are in specific places.
As of version 13.5.6, 142 cells and 1 core are available, with 1 cell in Yui's Ranch being unreachable.
100 cells and 1 core are needed for obtaining 10% Form Zygarde.

Red Essence

Red Essence is, according to Cairo, "[...] the residue found within Dens [...] left by Shadow Pokémon's aura. It is dangerous to all forms of life. Not just Pokémon.". It can be obtained from fighting and/or catching the Shadow Pokémon in Route 4's dens, and traded with Cairo for items, like crests and scents for lowering a Shadow Pokémon's heart gauge.