One of the thirteen men who fled into outlawry after the fall of Dorthonion in the Dagor Bragollach. Led by Barahir and his son Beren, this small band retreated into the highlands above their invaded homes, where they waged a secret war against Morgoth's forces for a time. At last, they were betrayed by Gorlim the Unhappy, and Hathaldir was slain with almost all the others (only Beren escaped death).
We know almost nothing else of Hathaldir's life, but it seems fair to assume from his title 'Hathaldir the Young' that he was the youngest of Barahir's outlaw companions.
Notes
1
The Elvish name-ending -dir can have various meanings, but most commonly it is a masculine agental suffix relating to an occupation or common activity. The hathal in Hathaldir's name is not explained, but it perhaps a variant on hathol 'axe-blade'. There was indeed a Númenórean later in history named Hatholdir, 'axe-man', and perhaps a similar meaning is intended for Hathaldir's name.