They lashed King Munmu's (문무왕) body onto a boat, rowed past jagged rocks, then tipped it into the cold East Sea. He'd told his court to bury him at sea so he'd rise as a dragon and guard Silla.
The scene comes from Samguk Yusa (삼국유사), written by Il-yeon (일연) in the 13th century. Munmu ruled from 661 to 681 and he'd spent his life fighting to weld the peninsula into Silla. In the book he gives this last, odd order, and the account paints a shockingly human picture: officials crying on the strand, the boat slipping away, the sea swallowing the king like a promise.
Samguk Sagi (삼국사기), compiled by Kim Busik (김부식) in the 12th century, barely flinches at the magic. It records the burial more soberly, which tells you the dragon part was already turning into a story people wanted. Centuries later the image stuck. Sailors and local priests kept tales of a dragon protecting the coast, and coastal shrines picked up the job of keeping the legend alive.
On the other side of the old Goguryeo world, Dae Joyeong (대조영) was doing his own kind of myth work. Chinese histories like the Old Book of Tang call him a rebel chief who, after Goguryeo fell, gathered Goguryeo refugees and Mohe tribes and declared a new kingdom in 698. Balhae's origin was messy and human, but later memory turned Dae Joyeong into a founder-hero who made order out of chaos.
Those two stories sit next to each other for a reason. After the fall of empires, leaders and chroniclers were trying to hold people together. Turning a dead king into a dragon was a way to promise protection. Turning a refugee leader into a king was a way to promise a future that made sense to strangers living together.
People back then picked myths that fixed fear into a story you could point to. Today we still visit the rocks, take the photos, and pass the lines along, because stories like that are better at holding a world together than any treaty ever was.