The Ur-Grue slashes you in the back for 48 damage. Since you regenerated a hitpoint before getting hit, you have 1/100 HP to the Ur-Grue's 156/215 HP.
Just kidding, dumbass. You actually got hit for 9999 damage, the maximum possible damage in Final Fantasy. You have been overkilled. The Ur-Grue takes your Megalixir and uses it on himself. You lose, and the Grue race becomes stronger.