The Battle of the God's Eye was one of the major battles in the War of the Five Kings, and a major victory for House Lannister. It broke nearly a third of the Reach's force, despite the Westermen being outnumbered, and gave Tywin Lannister the opportunity to attempt to break the Siege of King's Landing.
Tywin Lannister, intent on liberating his homeland from Robb Stark and the marauding Riverlanders, prepares ot march west when word arrives of Renly Baratheon's siege on King's Landing. Tywin immediately sets out to defend the capital, but Renly and his host are moving out to meet him. Tywin draws Randyll Tarly - who had been given command of twenty thousand men - away from the main host, which encamped south-east of Harrenhal, by the God's Eye. In the mean time, he and his main force moved into position around the unwary Baratheon camp.
As night falls, the Baratheon camp is unware of the oncoming battle. Renly constructs huge fires from the nearby wood, and his camp has a nearly festive attitude. Once the moon rises in full, however, horns sound from all directions, and the Lannister assault begins.
The Baratheon-Tyrell host attempts to form up, it's back to the lake, but their foes are already upon them. Fire blind and terror striken, many of the Reachmen fail to rally at all, dispersing in all directions. Those that stand, however, fight hard, despite most of them lacking armour or weaponry. King Renly is evacuated early by his Kingsguard, spirited down the Kingsroad and back toward King's Landing. This takes the heart out of his men, who break completely. The fighting loses all order, and quickly devolves into chaos. The Westermen carry the day, their momentum and discipline proving too much for their inexperienced and unprepared foes.
Considered King Renly's greatest loss, the Battle of the God's Eye would harm his reign in the early years, as many of those nobles who lost families in the battle saw his retreat as cowardly. The huge loss brought the armies of the Westerlands and those besieging King's Landing closer to equal, however, and led to Tywin being confident enough to engage in the Battle for King's Landing.