When
you summon Mimic, you can choose to summon it normally, or instead you can choose a
carriable artifact anywhere in the
realm. That
artifact is
banished, and the Mimic is placed at the artifact's
location instead as it transforms. You control the Mimic, and if the artifact did not
enter the realm this turn, the Mimic doesn't have
Summoning Sickness. It turns out that artifact was a Mimic in disguise all along!