Seems like a little weak for a legendary unless there's another effect.
Edit: to expand on the thought, it's giving up around 4 stat points from a vanilla minion to gain the effect. In other words it needs to attack and kill at least one minion to get up to par. And it has to be the one attacking, so it can't be abused with taunt-givers or anything to eat swarms of small minions.
As people said, it's Gruul 2.0 except, unless I'm missing something, it's worse because it gives up more stat points for its effect and does not reliably gain +2/+2 every turn.
Forget the stats, what's up with it being Legendary? Aren't legendaries named characters? What, did the Old Gods make it and say to themselves, 'hmm, what would be a good unique name? I know, Monstrosity!' ?
worse than Gruul with a more conditional effect...the only thing this card has over Gruul is that it isn't BGH'ed (if the 7 atk condition is kept) on the turn it is played...Gruul will continue to grow even if it doesn't clear the board while this card may just die after it hits one or two minions to even keep up with Gruul.