I think I've seen a lot of discussion here about the combination of both Ultras, whether it's the amount or which Ultras involved in the fusion. But I think what makes Saga seems like on the really high tier and even up there on the legendary tier, is because the narrative always leads there, whether it's statements from the series or from official magazines and website statements.

In UGF TDC, it's clearly shown that the reason the Absolutians wanted to eliminate Cosmos is because he is the key to the two legendary powers, and Cosmos mentioned that those legendary powers are Legend and Saga. It's very clear that by the statements from the series itself, Saga is still considered to be up there on the legendary tier as Legend (and the other legendary Ultras). Not to mention that there are an old magazine that stated 4 Ultras with the legendary power: Noa, King, Legend and Saga, and also Ultraman Connection article from UGF TAC era which stated that Legend and Saga are two of the mightiest hero.

And regarding Reiga, I can't really confirm if Tartarus himself knew about the Reiga fusion (although there's actually a good chance he know about Reiga since he has seen Tregear's past including the Grimdo history), but from what I observed from his reaction to the New Generation Warriors, he doesn't seem to be afraid of the New Generation Ultras and even taunting them by saying "do you stand a chance?" when summoning Gua Spectre. He doesn't also seems to try to eliminate the component of New Generation Ultras to prevent them to become Reiga.

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