If you shorten the damage part, you can fit in a little better description of the crumble ability for new players:
"Weapon: Deals 4 damage.
3 : Crumble
Deals an extra 10 damage, but self-destructs at end of turn."
I actually, VERY much like this card, since I've been wanting to see "disposable" weapons added for quite some time now.
It will make a very amusing addition to fractal decks (flying weapon + this + fractal).
Keep that combo in mind when balancing since it will likely be the most common method of use.
With that in mind, +10 / +15 damage may be a little too high, but I'm not sure on that. I would try and get in touch with some of the more experienced PvP players on the forums to get their take on it.
Other than that, I LOVE this card.
P.S. If you want I would like to make some art for it, Either 3D or modified stock photos.