The Arc of the Covenant

At the beginning of your end step, if you control
Arc of the Covenant but don't own it, sacrifice
another target non-land permanent you control
with the greatest mana value, mill 5 cards then
exile your graveyard. When Arc of the Covenant
enters the battlefield under it's owners control, creatures you control gain hexproof and indestructible until your next upkeep.
{5}: Target player gains control of Arc of the
Covenant.