Guard Daos is Gades, Amon and Daos combined into one creature that uses all of their abilities. It appears as the final boss in Lufia & the Fortress of Doom and as one of the final bosses in Lufia: The Legend Returns. Guard Daos is always fought after all of the Sinistrals have been defeated. Erim, the Sinistral of Death, has never been a part of Guard Daos. She was almost included in its first incarnation, but she broke free after she remembered the time she spent with her friends and joined them in the final battle against the creature. This would not be the end of Guard Daos, however, as the Sinistrals would resurface several years later. After being defeated by Wain and his friends, they fused together to become Guard Daos once again, but they were defeated and destroyed once more. As Erim was destroyed moments later, there was nothing that anyone could use resurrect the other Sinistrals, so Guard Daos was never created again. However, since Erim has been destroyed in Lufia II: Rise of the Sinistrals, in which the first encounter with the Sinistrals happens, and later returns in the other games, it is possible that there is another deity, such as Arek, who has the ability to revive the Sinistrals.
Guard Daos is the final boss in Lufia & the Fortress of Doom. It's fought after all of the Sinistrals have been defeated and they are all fused together in a final effort to defeat the heroes. Even though Guard Daos has all the powers of the Sinistrals (with the exception of Erim, who wasn't a part of Guard Daos), it was defeated and the Sinistrals were beaten once again.
It is interesting to note that some of Guard Daos' attacks were removed from the North American version of the game. Guard Daos is supposed to have the combined powers of all of the Sinistrals, including the strength of Gades, who uses only physical attacks in the game. However, since physical attacks were removed from Guard Daos' arsenal, it never uses them in the localization.
Guard Daos is one of the last bosses in Lufia: The Legend Returns. The creature is once again fought after the other Sinistrals have been defeated and resurrected by Erim, but even after combining their strength in order to become Guard Daos, they were defeated again and permanently destroyed this time.
Guard Daos takes on a much more robotic form in this game than in Lufia & the Fortress of Doom, and actually explodes after it's been defeated.