Idora is a Inga Horror and one of the nine Horrors freed from the slate that sealed the legendary Horror Eyrith.
Idora is freed from the Ady Slate when a mysterious person destroyed the slate using a Madou Art technique. Idora was the holder of Eyrith's seed, who possessed the body of a young woman. Idora is eventually slain by Raiga, but the Eyrith's seed she held quickly transfered into one of her fellow Ady Slate Horrors before Mayuri could seal it. Idora's essence survived because Mayuri's inner cage was left open, and she tried to persuade Mayuri into eating humans and becoming a Horror. Luckily, Raiga uses a Makai spell to enter Mayuri's consciousness to destroy Idora and save Mayuri.
Idora was a skilled enough fighter to give Raiga a hard time fighting even without being in her Horror form.
In the series, Granda is the 5th of the nine Horrors used as the foundation to seal Eyrith in the Ady Slate to be captured, and it was the first Ady Slate Horror to be the holder of Eyrith's seed.