Whenever Marit Lage's Slumber or another snow permanent enters the battlefield under your control,
scry 1
(Look at the top card of your library. You may put that card on the bottom of your library.).
At the beginning of your upkeep, if you control ten or more snow permanents, sacrifice Marit Lage's Slumber. If you do, create Marit Lage, a legendary 20/20 black Avatar creature token with
flying (This creature can't be blocked except by creatures with flying or reach.) and
indestructible (Effects that say "destroy" don't destroy that permanent. An indestructible creature can't be destroyed by damage.).