A third legend says that his older brother Biryu was his mother's son with Wutae but Onjo was born after the second marriage with King Dongmyeong.

When Yuri, born from Dongmyeong's previous wife in Dongbuyeo, came to Goguryeo and became the heir to the throne, Biryu and Onjo moved south to found their own kingdoms.

[6] According to the Samguk yusa, Biryu founded his kingdom in Michuhol (Korean: 미추홀; Hanja: 彌鄒忽), but his didn't last long.

In 5 BC, Onjo moved the capital city to a more defensible location south of the Han River, renaming it Hanam Wiryeseong, and sent a messenger to the king of the Mahan confederacy telling him of the recent action.

Biryu and Onjo were afraid that they would not be treated well by the one who had become crown prince, and so with Ogan, Mayeo, and some other ten ministers went south with a large number of commoners following them.

Biryu wanted to settle on the beach, and the ten ministers admonished him, saying "But the land to the south of the river: in the north it is bordered by the Han river, and the East occupied by high mountain peaks; the south faces marshland, and the west is guarded by the sea.

Onjo made his capital south of the river in Wiryeseong, and the ten ministers assisted him, and the country was called Sipje.

Then he returned to look upon Wirye, where the capital and its villages were stable as a three-legged cauldron, and the people dwelt in security.

After some time Jumong was not well-treated in Buyeo – this was during the second year of Jianzhao in early Han [during the reign of Emperor Yuan].

He married Soseono and she became his consort, and because she, in the creation of his realm, rendered great assistance, Jumong especially favored them and received Biryu and the other as if they were his own sons.

When Jumong was in Buyeo he had a child Yuryu by a woman of the Ye clan, and when he came he was made crown prince and the one who would succeed him.

Therefore, Biryu asked his younger brother Onjo, saying "Originally the great king fled the difficulties in Buyeo and escaped here.

Our mother's clan poured out the riches of their house to aid in the endeavor [of creating the kingdom], and her toils were many.

Now the great king has left this world, and the state has fallen to Yuryu, and if we were to follow along here it would be as depressing as a tumor.

The Pei Shih and Book of Sui both tell that one descendant of Dongmyeong was Gutae who was sincere in his benevolence.