There are two types of these cubes.
One has a max rank of epic and is tradeable.
The other has a max rank of unique and is untradeable.
The attendance event gave the tradeable one. I am wondering where the other, untradeable version comes from. I know you can get them from RA boxes and gachapon, but are there any more methods?
I looked up in KMS and it seems the bosses drop the tradeable version. I assume when they start dropping for us in GMS it will be the same.