This card can only be equipped to a Warrior-Type monster. A monster equipped with this card increases its ATK by 700 points. A Dragon-Type monster that battles with the monster equipped with this card is destroyed at the end of the Battle Phase. (Damage calculation is applied normally.)
Lightning Blade's downside doesn't really matter in practice. Oh no, you can't run Lightning Blade in Ice Barriers. That's probably a good thing, since it would suck there anyway. Blizzard Warrior and Legendary Fisherman II are like the only WATER Warriors in Duel Links "worth" equipping with something, and they're still not worth it anyway.