Revelations from the Announcement Showcase

[Enkidoh]

I agree. She also looks grotesque/monstrous enough to be a Terminus.

As for how, I'd say the same way primals are summoned in the modern day. A single individual may not be able to do so anymore, but many people + a source of aether + prayer can.

There's been plenty of examples of individual mortals summoning Primals in the present day with no assistance from others - Ysayle summoned/turned into Shiva one last time to freeze the Graiton with no assistance from the Heretics who were nowhere in sight (granted she did have a Crystal of Light to help her with that though), Gilgamesh summoned Enkidu on his own due to his loneliness (although that situation was part of the Hildibrand quests so the accuracy of exactly what happened then is very much in the air), Yotsuyu summoned Tsukiyomi entirelly from her hatred and pain towards Doma and everyone who had ever made her suffer. And then there is the WoL themself summoning bastardized versions of the Primals they had defeated themself during Eden, with not entirelly accurate results!

Even ACN's Carbuncles and the SMN's Egis and Demi-Summons are individual summonings, although those are not Primals and are not even really in the same league as them, but they are still 'summoning' in the strictest sense of the word. Really the only requirement for a summoning is a strong enough will and enough aether to do so - multiple summoners definetely help, but they are not a specific requirement, it just needs a strong enough will and idea to do so.