Witches Tower

Witches Tower enters the battlefield tapped.
As Witches Tower comes into play, exile target
human you control for as long as Witches Tower
remains on the battlefield, otherwise sacrifice it.
As long as you control Witches Tower, humans
you control have "This spell can't be countered."
{t} Add {c} to your mana pool.
{4}{t} Sacrifice Witches Tower, search your
library for a human knight, reveal it, put in to
your hand, then shuffle your library.