The Battle of the Seven Stars was allegedly fought at the early stage of the Andal Invasion. According to legend, it resulted in the fall of the First Men kingdoms of the Vale and the rise of House Arryn, and was thus the first great Andal victory over the First Men.
The battle itself was fought at the foot of the Giant's Lance, the highest mountain in the Vale. According to legend, Artys Arryn flew to the top of the mountain with a giant falcon and defeated the Griffin King in single combat.
With the death of the Griffin King, Artys Arryn was crowned King of the Mountain and the Vale, and the remaining First Men houses of the Vale bent the knee. Like much of Westerosi history, the battle may be a myth with some truth in it, or outright fiction.
In the books
The identity of the Griffin King is never outright confirmed. In The World of Ice and Fire, Maester Yandel suggests that the legend boils down to a glorification of the Battle of the Seven Stars, in which the historical Artys Arryn defeated King Robar II Royce and forced the First Men houses of the Vale into submission. Maester Yandel disputes that there were two separate battles, and that a Battle of the Giant's Lance predating the Battle of the Seven Stars simply never occured.
As both Robar II Royce and the "Griffin King" are presented as the last First Men kings of the Vale, it is likely that the latter is nothing but the mythical version of the former. Interestingly, although Artys Arryn claimed to have personally killed Robar II Royce, this claim has later been disputed.
If Maester Yandel's theory is true, neither the Winged Knight nor the Griffin King even existed; they are simply a combination of glorification of real events and myth. Maester Yandel further speculates that the idea of a large falcon derives from the Valyrian dragon riders.