[quote=Hebigami Shiho]
Sure, I don't mind. You can add her name in your sheet. I'll control her. :P
[/quote]

I feel that your Master character wouldn't intentionally summon a Servant like Nidhogg, so I'm thinking we have her trying to summon the Eagle with a piece of Yggdrasil that survived Ragnorok. However, it was a root, not a branch, so it summons Nidhogg instead.