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The ered'ruin are a demonic species that contains the doomguard. The doomguard have also been stated to be of the "doomguard" race. As Doomlords are part of doomguard species they are Ered'ruin as well.
Related to the eredar? Edit
The ered'ruin may include races such as fiends and daemons as well. They may also be related to the eredar as suggested by the use of the term "ered" in their name. However this could be false, in contrast to the Ered'ruin, the Eredar are not pure demons like the Ered'ruin are, rather just corrupted by Sargeras.