If you don't know, you do not gain full Mana Crystals. The only other card that sets mana like this is Mojomaster Zihi. If Zihi were to give Mana Crystals, they would be empty (confirmed), and I assume Nozdormu the Timeless uses the same mechanics. It's not the same as Astral Communion since that's "Gain" Mana Crystals.
Assuming set means 10 full mana crystals not 10 mana slots, Ive been trying to think of a way this is good. And there are some situations but they require you making your deck suck if you dont draw it or have no follow up when you play it, losing any tempo advantage it may give you.
kudos to blizzard for trying to push this big paladin meme further but I dont think this is it chief.
discovering this card is great though, its a mana refresh for 4 on an 8/8 body
I'm probably completely wrong, but I just thought it might be what control-oriented Paladins need right now. Paladin has some good lategame combos and stabilizers, but many of their cards are just so expensive that they fall behind too often.
Of course it's a very risky move to just give your opponent 10 mana to overrun you, but otherwise, the alternative is to struggle until you can play your Equality or Shrink Ray with a Wild Pyromancer, or hope you last until you can play Crystalsmith Kangor with a heal effect. This allows you to move the game to a stage where you are more capable of dealing with your opponent's moves. It even makes Nozari somewhat more playable.
The way I see this card, draw it early and it works with your deck, draw it late and it gives you a nice temp for 4 mana. People might hate the give your opponent 10 mana, but you wouldn't play this unless you already have board control. I love it, aggro decks will get melted as big minions easily take over, drop this into a Nozari for a recovery or an 8 mana clear.