{5}: At the beginning of your next upkeep, choose a basic land type. Giant Slug gains landwalk of the chosen type until the end of that turn. (It can't be blocked as long as defending player controls a land of that type.)
If the Slug changes controllers after the mana is spent, the player who activates this ability selects a landwalk during their next upkeep even if they don’t control it at the time.
When the ability resolves, it creates another ability that triggers at the beginning of your next upkeep. When that ability resolves, you choose a basic land type.