Keep flipping until you get tails
Every time a special Umbreon card is released, it gains value like wildfire spreads
I dont think this is really going to fly but combining this with Oblivion Ring, Leonin Relic-Warder, and related effects leads to a four card combo that will mill the opponent out on the spot using only cards that have decent utility all by themselves (other than the Altar, of course)
With just two in each set, and with them being even rarer than Enchanted cards, these are likely to remain valuable for some time to come
As if that somehow wasnt enough, theres also the Cosmic Foil variant, which is even rarer than most serialized cards
To keep your collectables looking their best, you should place them in a dark and cool place