Void Chests are found in the QUESTS tab of the RAIDS section.

It contains 2 chests:

VOID CHEST (I) can be opened for 30 Void keys

VOID CHEST (II) can be opened for 100 Void keys (II)

Void keys (I) and Void keys (II) can be obtained by defeating various Bosses in Raids mode.

IMPORTANT: There’s a certain chance of getting abilities and special moves from Void Chests—this means that there’s no 100% guarantee of getting them. To find out more, click on the chest you are interested in, then on the "i" button to the right of the chest, followed by the "i" button in the upper right corner. The description will tell you the chance of getting abilities and special moves.