Like most Magic: The Gathering cards, the value of The Darkness Crystal depends on a few consistent factors: its rarity, the printing and finish (regular or foil), its physical condition, and the set it was printed in (Final Fantasy)
Its simple: One mana to reanimate a creature with the only actual drawback being the life lost in the process
Trostani being legendary hasnt hurt the five-mana creature as much as I thought it would because your opponent will assuredly have to remove it from the battlefield to be able to win the game, which makes having a second copy in your hand less of a drawback
Cards like Leyline Of The Void that exile opponents' cards sent to the graveyard are fantastic too, since they won't be able to get the destroyed lands back from the rest of Fall Of The Thran
This aging program isnt perfect, and might not be suited for every kind of player, but its always provided an incredibly reliable way to play this spectacular card game