M'KRAAN INFO
As the Nexus of all Realities, the Crystal can allow travel between universes, but opening the Crystal could cause the mass of the neutron galaxy inside to suck in the entire universe and cause a chain-reaction across the Omniverse, destroying all universes in existence. The city inside can only be reached once every million years: when specific galaxies are aligned, the M'Kraan Crystal will allow entry for a short period of time. It also houses the White Hot Room, the place in which the Phoenix Force places all beings to heal them or return them to life. As it is a singleton across all universes, the reality-hopping Exiles have recently figured out how to use the Crystal to visit selected realities
COSMIC CUBE INFO
The Cosmic Cube is actually a containment device created by various civilizations throughout the Marvel Universe at various times, including the Skrulls (creators of the Cube that would eventually evolve into the Shaper of Worlds), and various other, unnamed civilizations (whose Cubes were gathered/stolen by unknown means by The Magus in the Infinity War story arc and The Goddess in the Infinity Crusade story arc). These matrices—which may or may not actually be shaped like a cube—are suffused with reality-warping energies of unknown composition that comes from the realm of the Beyonders.