Cassara, Soul-Razor

When Cassara, Soul-Razor enters the battlefield,
gain control of target creature or planeswalker for
as long as you control Cassara, Soul-Razor.
{x}{u}{b}, Target player loses X life and discards X cards, and you gain X life and draw X cards.
Look at the top X cards of target opponent's
library. Exile one of those cards and put the
rest back on top of that player's library in
any order.