Keravan, Lore Seeker

Kerevan, Lore Seeker doesn't untap during your
untap step.
{t} Search your library for an artifact or a creature
card and exile it. As long as Kerevan is tapped,
you may cast cards exiled with this ability.
Sacrifice a nonland permanent: Untap Keravan,
Lore Seeker.
"There are ways to learn that do not require a library.
You just have to be curious."