Giant Slug

{5}: At the beginning of your next upkeep, choose a basic land type. This creature 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.)

Chronicles Foreign Black Border

Chronicles

o5: During your next upkeep, choose a basic landwalk ability. Giant Slug gains that landwalk ability until end of turn.

Legends

o5: During controller's next upkeep Giant Slug gains landwalk ability of controller's choice until end of turn. The type of landwalk chosen must correspond with one of the five basic land types.

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.