Altar of eternal knowledge

Altar of eternal Knowledge comes into play
tapped. Untap only if you control three or more Planeswalkers.
{t}, Sacrifice a creature. Target Planeswalker you control may activate up to two Loyalty abilities
until end of turn.
{t} Sacrifice Altar of eternal knowledge, prevent
all damage to Planeswalkers you control by sources
your opponents control until end of turn.