To be clear, imagine you have 10 mana, you play the ring, you then have 6 mana stored for your next turn (since the 1 durability already on the ring is added). If you only have 1 Durability left and your spell costs 2 mana, it will use one Mana Crystal. I think 6 mana maximum is a fair amount and not often so easy to do but I like the idea as a one of in Freeze mage.
Some may look at this and say "OP, OP", though if you compare it to Klaxxi or Master of Evolution, it really isn't and is actually probably a bit worse than those two. If you have any unspent mana, you can either play a card, Hero Power or let this guy grow. This guy is also really useful if you have spare mana but no cards to play.
Flavor-wise, the Arcanist is sucking all of the power of your unused mana and is gaining power from it.
EDIT: Changed "end of each turn" to "end of your turn".
Akondro feeds Bananas to your Beasts and has many synergies with existing cards. For example, play Akondro with Tundra Rhino in play, smash the Emperor into something to trigger the Deathrattle, then flood your board with six Bananas (one for each mana spent to play Akondro)! You can also trigger it with Feign Death or Princess Huhuran. If Akondro dies on your opponent's turn, and your opponent played a Beast on that turn prior to the monkey's death, you get the Bananas bonus too!
The classic "Spend Resources, get them back if target dies" Warlock Spell all the way back in Vanilla
3 damage for 5 Mana is obviously very bad, but if you kill your target with it, then it's free. Late game when you have 10 mana crystals, you may even use this to actually gain Mana Crystals, provided you can spare 5 Mana first!
Can only be played from turn 10 onwards but will be "free". Compared to Sea Giant which can be played on as early as turn 5 or 6 from zoo players, Seething Giant is a much slower card but guaranteed for play from turn 10 onwards.
This is an old card design I reworked some days ago, and when I came to check out what would be the theme of this week's competition I just couldn't not submit it. She's basically the Clockwork Card Dealer (did I get it right?) Tavern Brawl in a card. Could be crazy in Ramp Druid and maybe some tempo decks.
Insanely good for control decks that spam the hero power. Even if the hero power switches, it still costs zero (see Frost Giant). This means that this is also useful in things like handlock, enabling draw 1 card, become Jarraxus, and summon an Infernal in one turn. THE VALUE!
He's still waiting for a promotion, and figures if he just works a little harder...