It grew on me a bit after brainstorming some other fairly basic ideas (I am distinctly not creative, so I'm basically permuting together story hooks or characters and checking if they relate to death/the underworld, Pseudo-Servants, or Sumerian mythology)
- It didn't really make sense to say that Kur was related to the Demiplane of Dread, even if they share some thematic similarities. For some reason the entire "Dreadlords are cursed in a terribly ironic way" thing seemed to mesh with how Ereshkigal keeps trying to improve Kur and failing.
- Orpheus-ing Laeticia seems too trite, and there's no reason to stick her in the Mesopotamian afterlife. She also probably doesn't make a reasonable Pseudo-Servant host for a completely different Servant.
- Pseudo-servant Fiore... might work? Feels simplistic, but Spiritual Evocation can be loosely related to death, even with the lack of canon detail, and her personality doesn't seem entirely incompatible.
- I have no idea what's going on with Kara from Melancholy of One, since it's been too long since I read it, I haven't really seen her used elsewhere, and I don't know if she's available. She has a deep relationship with death that I thought might be interesting.
- Killing Godafrid or having a near-death experience and making him a Geistcaller gives him a connection with death, but isn't really a novel concept
- Erik's use of obsidian mirror shards from an Aztec death goddess are death-adjacent, but that's not really enough to make it a plot hook. Shiki is in the same boat.
- Saying "Hanali did it" requires at least a bit more explanation. It can lubricate other explanations, but not create them out of whole cloth.
- Saying "Zelretch did it" opens a ridiculously huge space of possibilities. The lack of constraints makes it difficult to consider with my aforementioned lack of creativity
Dropping the Sword of Atli into the Age of the Gods seemed incredibly simple, but the relationship between Mars, Sefar, and the Age of the Gods actually fit together a bit, thematically. The problem, as I mentioned, is that it's creation myth, so has ridiculous power level and is a bit... sacred? to be interacting with. That said, there's precedent for having differences in that part of pre-history. It's also potentially more temporal paradox weirdness, and would be a bit of a stretch to relate to Ereshkigal specifically.
Alternatively, the relationship between the Sword of War/Mars and the Goddess of Victory/Venus (or just any treasure and Mankind's Oldest Hero) seemed at least thematically coherent, albeit mixing metaphors. Heck, the FGO Christmas event that featured Ereshkigal had Altera the Santa in it.
This is abstract enough it might fit better in the Grail Works general discussion thread, though.