User blog comment:Jopjopjop/Volume 20 Spoilers/@comment-16281343-20150719004716

What the fuck is wrong with Nidhoggr?! He is fodder level compared to the other legendary Evil Dragons. One [Solid Impact Booster] from Issei and his face is fucking broken already.

Rias' Crimson Extinct Dragonar :

It is the combined technique of Rias and Issei that was mentioned in v19.

This combined technique grants Rias a Scale Mail of her own.

Issei uses his Wyverns and they sync both of their auras. Once synced, the wyverns attach themselves to Rias to create a female version of the Boosted Gear Scale Mail.