Balance (X): You get the effect if you play this card when you have exactly (X) mana left.
This requires the player to think about the most optimal way of playing his/her cards to get the maximum out of them.
Arcane Prophet is a decent 3 drop but if you play him on turn 4 you might be able to immediately follow it up with a 1 mana damage spell. If you need the Spell Power later than you will have to spend mana until you have exactly 4 left when you play it.
Last Light would allow for some nail biter combo decks. You can decide if you would rather draw 4 and try winning with 8 mana this turn and 10 on your next, or you might think you have a better chance if you play it when you have 6 mana left and get two more cards, but only 4 mana this turn and 10 on the next.
Lord of Carnage is more straight forward. A 5 mana 5/5 Demon, or if you play him with exactly 9 mana left then he is a 5 mana 9/9.
2
I actually crafted Thalnos two years ago and I never regret this. Thalnos is a classic card though.
2
I'm talking about the Splash movie with Tom Hanks and Daryl Hannah where she is a mermaid and choses a name - Madison.
5
http://i.imgur.com/2b1fzt0.png