“I see you are lamenting over turning your blade to your mother, but you must get over it. You couldn't help it, the Blade of Darkness was controlling you. But now that I've removed the curse, you should take it.”
—Saias talking to Mareeta in Thracia 776
Saias (サイアス Saiasu, Cyas in the Japanese version) is a playable character in Fire Emblem: Thracia 776. He is the illegitimate son of Arvis and Aida, and grandson of Cowen. He is a priest of Velthomer and one of the greatest tacticians in the history of Jugdral. He is kind and friendly to everyone he meets. As revealed in Chapter 22 of Thracia 776, Aida died protecting Saias from Manfroy, who sees Saias as a potential threat because of his Fala blood.
After Mareeta fell victim to the Darkness Sword, Saias removed from the curse from the sword, and afterwards, in Chapter 12, he got Mareeta to get over fighting Eyvel, since she was not in control of herself, and to not hate the Darkness Sword, and since Saias removed the curse from the sword, it becomes Mareeta's Sword, a powerful personal weapon for her. In Chapter 23, Ced gives Saias the Blaggi Sword to give to Leif.
It is also mentioned in-game that Saias has Fala blood which may be seen as confirmation that he is truly Arvis's son. Considering that Saias' half-siblings Julius and Julia have major Loptyr and major Naga respectively, it is a fair assumption that he is the child of Arvis that inherited his major Fala blood as well as the Valflame, which would make him a candidate for ruling Velthomer.
Saias manages to make Chapter 17A and Chapter 22 absolutely hell for you, but he does join you at the beginning of Chapter 23. Basically, you have to choose between him and Ced, and Saias cannot even compare to Ced when fighting, and the only advantages he has is his leadership stars as well as his useful staves.
If you decide to use Ced instead of Saias, remember to take his three staves off him (a Heal, a Warp Staff and a Silence Staff).
Saias - The Bishop of Crimson Flame (赤き炎の司祭 Akaki honoo no shisai)
Saias returned to Velthomer and helped restore the trampled land. His talent was later recognized by Seliph, and he went into the service of the New Kingdom of Grannvale, where he contributed greatly to its reconstruction.