Pretty bad, if it isn't discarded it's 4 mana deal 4 damage which is kind of bad considering soulfire, darkbomb, implosion. To make use of the discard mechanic, you have to have a card that discards in your deck, and this card, and then you have to draw them both at the same time, and then you have to hope you discard this card and not something else.
This is a card that seems pretty meh but it has such a unique mechanic to it that it will really need to be tested first before we can say anything definitive I think. Cards like that are what make expansions. Very cool.