Ipnu Rivulet

Oracle Text

T: Add C.

T, Pay 1 life: Add U.

1U, T, Sacrifice a Desert: Target player puts the top four cards of their library into their graveyard.

Card Rulings

4/18/2017 Desert is a land subtype with no special meaning. It doesn’t grant the land an intrinsic mana ability. Other cards may care about which lands are Deserts.
7/14/2017 If a Desert has an ability with a cost of “Sacrifice a Desert,” you can sacrifice that Desert to pay the cost for its own ability.