The king of Eleusis, Cercyon, was a strong man who challenged those passing through his lands to a wrestling match, passing through the roads around Eleusis. Although he promised to give his kingdom to the winner, the losers weren’t safe – they were killed. As you can imagine, there were never any winners! Theseus defeated the king skillfully instead of using his physical power and captured his kingdom after killing him.
The son of Poseidon, Procrustes, controlled the path between Athens and Eleusis on Mount Korydallos, Erineus. He invited all those who traveled his way to help him with a blacksmithing task, which often resulted in the deaths of those who tried to help. Instead, Theseus beheaded him along with cutting his legs with his own axe.
As you can see, Theseus underwent man challenging on the way to finding King Aegeus in order to learn who his father was. In the end, Theseus ultimately learned that both Poseidon and King Aegeus were considered to be his father.