This special crystal mirror is able to form a connection between two dimensions. As far as we know, only a pony crossed it to reach a dimension parallel to Equestria. This also allowed to demonstrate a small disadvantage (or advantage depending on where you look): the mirror changes the traveler's body, transforming it into a different creature. That was enough to accomplish Stalliongrad's goals.
After locating the Portal in the Crystal Empire thanks to an intense network of spies, the Supreme Soviet authorized the sending of secret agents to be able to gain possession of such a magical wonder. The mission ended in success. Although the entire artifact could not be stolen, some agents discovered some scrolls that contained the studies done by the most illustrious Tecno-magicians of Equestria. The mirror is made up of [REDACTED] a special rare crystal found in the region of [REDACTED].
After an expensive expedition to procure the crystal [REDACTED], Stalliongradian scientists set to work to build their own interdimensional portal. After just two weeks of hard work, Stalliongrad's personal portal was finally finished. Unfortunately it was large enough to pass a small group (max 10) of ponies at the same time, but it was still a great success for the nation. Now that portal can be used to explore the infinities of the Multiuniverse.
A further discovery was that thanks to small fragments of the [REDACTED] inscribed with runes, the traveling pony can maintain its appearance even once it has crossed the Mirror, or, after having crossed it, maintain some peculiarities of their original body, especially for unicorns and pegases. Although they will be weaker if not fed by due magic.