Oracle Text

As Shimmer enters the battlefield, choose a land type.

Each land of the chosen type has phasing. (It phases in or out before its controller untaps during each of their untap steps. While it's phased out, it's treated as though it doesn't exist.)

Card Rulings

10/4/2004 Can affect basic or non-basic types, but it must be for a specific type. The chosen type must be an existing land type. See the glossary of the comprehensive rulebook for more details on existing land types.