Dire Goose King - Large Monster

You cautiously move along the tunnel, one hand on the hilt of your sword, the other on the tunnel wall. Ahead you see a faint light and hear a light splashing noise, followed by the sound of metal scraping on rock. As you move into the light, you find yourself in a large, open cavern with a deep pool of water to one side. At the far end of the cavern you make out a large, feathered shape - surely an owlbear? Roll for stealth... oh, a 3? You accidentally kick a small rock, which splashes into the