That's the thing, there aren't very many resources on them. I know a lot of private/personal stuff goes on with lesser known entities like this. But I haven't found many actual sources. That's where I've found personal experiences and experiments are so important. The Hydra is a strong force. I managed to sync up to him by scrying into a picture of him like it was a sigil. I've done the same with the Mester Stoor Worm. There's a presence, but it's much more subtle. Nidhogg and Jormungandr have pretty strong followings in the Thursatru and Roknatru communities, so those would be good places to research them, and the Eddas. Though, I have found literally nothing about the other Grafvitr. They were only mentioned once, in a list in the Eddas, and everyone seems to ignore them. But they were called great serpents, just like Nidhogg and Jormungandr. I have a feeling Nidhogg would be willing to introduce practitioners to them. There's always the Slavic Veles, or the Vedic Vritra. Or, the biblical Red Dragon himelf. I've worked closely with Behemoth in the past. I can post the sigil he gave me here a little later. He'd be easier than the other lesser known serpents to contact. The anti-cosmics have a bunch of writings and rituals regarding Chaos serpents too.