The Sea Above the Sky

At the beginning of your upkeep, you may target a permanent an opponent controls with mana value X. If you do, exchange control of it with a permanent you control with mana value at least X. Then you lose X life. If the chosen permanent is a token, X is equal to the highest between its power and toughness. If the chosen permanent is a land, X is considered as 5. If you don’t activate this ability, sacrifice The Sea Above the Sky.