Interloper is a catch-all term for beings who make invasive moves into our reality from other universes. These are often beings who are neither human nore actually good people. The methods of travel are varried, often ranging from the use of an artifact or contacting a service to bring themselves through the divide. A popular service for interdimensional travel, which is not available to inhabitants of this world, is a sort of taxi service where a host of your choosing can ferry you to other universes. There are many people who are in the employ of this service, making the selection massive. Those caught infringing on private contractors in terms of uniform design are killed and then fired.
Many artifacts that transport people to other dimensions are often single use, otherwise, they only operate on a round trip basis. Common items include a Silver Key, Warp Generator, Decoherence Field Generator, or a Shining Trapezohedron.
Since the theorized structure of the multiverse is one of millions of bubble universes, ripening and rotting like fruit, it is safe to assume these items somehow allow for one to break out of the core and through the skin to traverse the space between and enter another universe.
Here's an example of invasive species from alternative realities.