The Power Stone

This card starts in exile and cannot be brought
back but by the following methods.
If a "Ronan, the Accuser" Card is in your
graveyard, you may sacrifice a "Nova Prime, Xandarian Leader" card to obtain the Stone.
"Starlord, Legendary Outlaw" can also collect this
from the Temple of the Power Stone.
If "Thanos, the Mad Titan" uses this card, you
can deal 3 damage to any creature or Planeswalker