Sword in Exile Series by Ann Marston

An epic medieval saga details the adventures of Gareth--the last of the lords of Skai--who is summoned back to his conquered land, only to discover that the magic of the standing stones is lost forever and that the Rune Blade is now in the possession of evil.

In this second book in the Sword of Exile trilogy, spring holds promise after the Somber Riders' long rule over the Celea lands. Destiny pulls together a girl whose Rune Blade thirsts for justice, a brash young sovereign whose heart seeks revenge, and a renegade Somber Rider, touched with Tyadda magic, who remembers a long-forgotten dream.

After generations of exile, the Celae get a chance to return to their island home when twin princes--Rowan the sorcerer and Acaren, the destined ruler--join forces with their soulmates to venture to the forbidden isle to find two mystical swords, Heartfire and Soulshadow.