Firebringer, book 3: The Son of Summer Stars
Meredith Ann Pierce2003

A secret haunts Jan's dying father, Korr a secret so dark that, if exposed, could bring the downfall of the Firebringer. Jan is left with its terrible burden, and an impossible choice: renounce Tek, his beloved mate, and lose forever the kingship of the unicorns, or return and face the consequences of his father's acts, and of his own.