|Although this article is based on canonical information, the actual name of this subject is pure conjecture.
The Greyjoy fleet unexpectedly met Tyrell longboats at the mouth of the river Mander. Lord Quellon died in the fighting, making his son Balon the new Lord of the Iron Islands. Balon ordered a tactical retreat, and thus the Greyjoy participation in Robert's Rebellion ended.
In the books
The battle is not mentioned in the A Song of Ice and Fire novels. According to The World of Ice and Fire, Quellon Greyjoy led fifty longships against the Reach. At the mouth of the Mander, they were surprised by the longboats of the Shield Islands. The ironborn won the battle, but Balon retreated back to Pyke nonetheless.