Both cards have incredible combo potential to end the game with and are just good value in decks that want to use them fairly
That means if you add a green mana to your mana pool during one step or phase, you can spend it during a later step or phase, or even a later turn
This planeswalkers +1 ability is awesome at making your creatures better, and the -3 ability is also quite good if you only need a card like Perilous Snare to take control, instead of waiting for the final ability
The dash cost is also going to be of interest, especially as a finsher, and it should be noted that the commander's discount effect also applies to that casting cost, making it cost 4 as well, and that with 4 power that also triggers the enchantments that cares about that
Based on the latest market prices tracked on TCG Stacked, The Darkness Crystal is worth about $10.39 USD for a regular copy and $12.97 USD for a foil copy