Thranduil is Legolas’s father in J.R.R. Tolkien’s The Lord of the Rings.
Thranduil is the Elvenking who rules the Woodland Realm in northern Mirkwood. In The Hobbit, he appears as the king who imprisons Thorin Oakenshield and the other Dwarves after they enter his halls, although he later takes part in the defense of his realm and the Battle of the Five Armies.
Legolas represents the Woodland Realm in the Fellowship of the Ring. Tolkien’s main text identifies him as the son of the Elvenking, while later reference material and adaptations make the father-son relationship more explicit for audiences.
Thranduil is distinct from Elrond and Celeborn, two other prominent Elven rulers. Elrond rules Rivendell and is Arwen’s father; Celeborn rules Lothlórien with Galadriel and is not Legolas’s parent.