The Power stone

Shroud
{4}: You gain control of this artifact. Any player
may activate this ability.
{x}, {t}: Copy target instant or sorcery spell with converted mana cost X or less.
{x}, {t}: Spells you cast cost X less to cast until end
of turn. Only activate if you control 3 or more Infinity Stones.
{x}{x}, {t}: Cast up to X spells without paying their
mana costs. Only activate if you control 5 or more
Infinity Stones.