War of Wrath
Thranduil is one of the Sindar or Grey Elves who speak Sindarin as opposed to Quenya, the language spoken by Noldorin Elves like Galadriel. The name "Thranduil" means "vigorous spring" in Sindarin. Following the end of the First Age and the destruction of much of Beleriand during the War of Wrath, many Sindar migrate into the east of Middle-earth. Crossing the Misty Mountains, they found populations of Silvan Elves living in the woodlands that bordered the River Anduin. The Sindar were welcomed by these people, and some were made leaders and rulers over them. In Appendix B of "The Lord of the Rings" Tolkien states that Thranduil was one of the Sindar who migrated eastward early in the Second Age and established kingdoms among the Silvan Elves. Later writings in "Unfinished Tales" make Thranduil's father Oropher the founder of the realm, which originally included the southern portions of the forest of Greenwood the Great, later known as Mirkwood.