I don't think this would be a very functional card, unless you were playing with Illusion decks, or Nature decks which can summon rares. But in the Nature deck case, you are already inflicting enough damage, so you don't really need Dragons as well
In Illusion decks it could be useful, but Illusion requires a great amount of gems, so it's not really a good idea to combine them with Dragons.
In either case and in any other deck, this card would inflict averagely 30 damage, which is kind of poor for a rare Dragon which discards all your rares once you play it and it costs 25 recruits.
Perhaps increase the coefficient of damage inflicted (double it?) or add a stock increase?