It used to be that if you cast a cascade card, such as Shardless Agent, that you could then cast either side of a card like this
The difference is that while they have an ink cost on them, there is also an extra block of text in the description area of the card which details that you can exert a character (turning them sideways and opening them to attack) of a minimum cost in order to play the song card for free
Its level two ability, which costs two mana, lets you do two damage to each opponent whenever you discard a card
Cards like Thundermaw Hellkite are fantastic threats too, and great finishers, but they rarely do the whole job, they rely on other cards having done a lot of prior work
This is a straight-up powerful effect
There have only been 121 cards given that rating