Lucian Kos, Orzhov assassin

Shroud First strike Attune with Kingslayer (when Lucian enters the battlefield its owner may put Kingslayer from their library into their hand and shuffle their library)
Lucian Kos enters the battlefield under target opponents control
Lucian Kos cannot attack its owner
Whenever you cast a spell mill two cards
{3}{u}{b}: exile Lucian Kos and return it to the battlefield transformed under its owners control.