Scion of Marit Lage

Whenever a 20/20 with flying and indestructible enters the battlefield under your control, Marit
Lage's Scion becomes a copy of that creature.
{b}{b}{b}, sacrifice Marit Lage's Scion: Search your library for a card called Marit Lage's Slumber or
Dark Depths, reveal it, put it into your hand,
then shuffle your library.
"I knew it! I should've looked in the sewers since the beginning."