|Name in Japanese||オストルム|
|Voice Actor (Japanese)||Masami Iwasaki|
|Voice Actor (English)||Justin Cook|
Ostrum (オストルム Osutorumu?) is the fourth original Dragon sent to Earth from Thanatos. His introduction is a violent one, as he destroys the ISDA's L-3 space station in his first few moments in Earth's orbit. He has no mercy for humans, but his top priority seems to be the destruction of the Earth-made Dragons. He is able to take Communicator form, but who exactly he resonated with is unknown. However, the only known direct human contact he had before taking his Communicator form was with Yuuya. Later, it appears that Ostrum dies in an explosion caused by the Type-X warhead. However, he is revealed to have lived in the final episode, but then dies after he is defeated by Gio.
Ostrum is a fire type Dragon. He is able to create bursts of flames, and is also able to incapacitate his opponents with blinding lights. As an original Dragon, he is an extremely formidable opponent, and is capable of overpowering the entire Lindworm Unit.
Thanatos is shown to be able to produce billions of Dragons resembling Ostrum.