Cost - 3
Rarity - Legendary
Faction - Voidborne (Seems like the closest related faction)
type - Minion
polymorph (5)
What is polymorph? when a creature with the polymorph keyword is played, It transforms into a random creature card in your hand that is equal to or less than the polymorph keywords value. In this example since its polymorph 5, the mimic will transform into a random minion card in your hand that costs 5 or less.
If the mimic cant polymorph for whatever reason, its base self is spawned from the card instead. It would look like your typical fantasy mimic, probably a chest with teeth. If you do spawn its chest form It would probably have the same stats as other 3 cost ground tanks. If the mimic didnt polymorph into a random creature and is its normal self it counts as all faction types
Let me know if you need anything clarified because I'm not the best at explaining things. Also feel free to suggest anything. I know this may not be the most balanced card, but my idea is mainly the concept of a mimic type creature and the polymorph ability than the balancing behind it.