The Omni Cube was an ancient device in possession of Sirius that was stolen by Altair, during the events of Bomberman 64. The cube was powerful enough to trap entire galaxies into his interior, making his user almost invincible. The cube appeared to be made of unknown materials and could not be destroyed easily.
The galaxies trapped there could not be released, unless it's user dies. Altair used the cube powers to make himself more powerful, learning all the secrets of the civilizations he trapped. Also, he could completely control the lives within the cube, making himself a kind of god for the trapped essence in there.