Finally.
Okay, so most of chapter 14 is about you going through Irkalla. There are a series of gates, and at each one you're asked a question. Answering properly does seem to make the battle easie; I tried experimenting on one choice, and the "wrong" option put me up against a green oni and the "right" option put me up against some skeletons.
As for the actual plot, none of your major teammates can follow you to Irkalla because being in the afterlife is apparently a "minus" to gods and heroic spirits. Ishtar gets you there by just blasting a hole in the ground or something at Kutha (Ereshikigal's main temple). You see some spooky spirits as you go along, and eventually come across the first gate of seven.
Ishtar explains how you need to go up against a trial, and then the gate calls you (by name, which Ishtar finds strange). The question is, paraphrasing, "Who is more beautiful, Ishtar or Ereshikigal?"
Ishtar is very annoyed.
Upon finishing the battle, you pass through the gate and Ishtar gets zapped. In the original myth she had to lose a piece of clothing with each gate, but in F/GO she just shrinks.
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The second gate asks, again paraphrasing, "Say you came across a large stash of precious gems. Who would you entrust with the task of safekeeping it?", with the choices again being Ishtar and Ereshkigal.
When you reach the third gate, you find Gilgamesh, who is very amused to see a shrunken Ishtar. He exposits about Irkalla and I think Mashu asks him why he never tried going down here to rescue Enkidu. I don't remember Gil's answer, but I think he actually couldn't for whatever reason. Anyway the question this time is roughly "Think of a battle companion. Someone you believe in, adore, and attain victory together with. Who comes to mind?"
Your choices are Ishtar or Gilgamesh, and regardless which you pick it goes straight to the battle.
I don't remember what happened at gate 4. The question was "Humans desire only blessings from the gods, and thus devote their lives to them. Now answer: the fastidious goddess who deserves such effort is:", and the choices are again Ishtar and Ereshkigal.
At gate 5 (or maybe it was gate 4, I don't remember), Gilgamesh talks about how Ishtar and Ereshkigal are completely different. He gives this long story about some stupid stunt Ishtar pulled, and all you can really comment is that Ishtar sure is a free spirit. Then he talks about how Ereshkigal spent the entirety of her life trapped underground in Irkalla, with only the spirits of the dead as company. Anyways the question this time was kinda weird:
"Existence revolves around both minimizing and maximizing waste. As living beings, it is only natural to eat splendid meals, come up with ingenious schemes, and view amazing delights. But note: If you consume without end, you'll grow fat. In that case, the person you should entrust your life to is:"
In what I can only assume is some kind of pun, the choices are "Oil, Meat, and Ishtar" and "The gentle livered Ereshkigal".
Afterwards, the game skips the last few gates.
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Oh yeah, after Gilgamesh talks about Ishtar and Ereshkigal he theorizes the reason he died was because Ereshkigal wanted to force (you) to visit her.
After you make it to the heart of Irkalla, Ereshkigal dramatically appears and makes the pressure to drop to 500 hecto pascals, which is half the usual sea level pressure. Roman urges you to defeat her before it gets any worse, but then she decides to reveal her ~true form~: Extra Rin.
Ishtar, Gilgamesh, and Mashu are stunned, but you are completely unsurprised. Ereshkigal can't understand why, so you explain that you already knew. I mean, you've already chatted with her four times. Ereshkigal is floored, because apparently she didn't know that she was talking to you in her true form. I guess this is why she was surprised you weren't scared of her, because she thought she was a giant spooky ghost.
Ereshkigal grabs her head and starts rolling around in embarassment, and Mashu asks if you should take the chance to attack. I guess (you) feel bad for her, so you tell Mashu to just wait.
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Ereshkigal quickly recovers and gets serious. The way she sees it, you're doomed either way. Either she kills you now, or you die in two days when Gorgon comes back. Her goal is to conquer Uruk, attain the Holy Grail, and drop all of humanity into Irkalla. I get the feeling the implication is that this is how she intends to save her worshippers.
She tells you that if you decide to interfere, you will die too. You can then briefly break her poise by calling her "Ere-chan".
Anyway Gilgamesh notes that even if he might be dead right now, he's still royalty so he has the right to judge Ereshkigal for her treachery. He has a question for her: why did she join the Three Goddesses? Why did she reject her duty of protecting the Sumerian nation and cuture?
Ereshkigal thinks that's a stupid question, and declares her duty has never changed. As the being in charge of the afterlife, her reason for existence, her destiny is to confine every soul in existence in Irkalla. So if Gilgamesh intends to stand in her way, she is entirely ready to rain down some divine punishment. Gilgamesh is impressed with her speech, and declares there is no longer any need to ask any questions. He'll have her head, as simple as that.
Ereshkigal thinks Gil is being ridiculously overconfident, but before she starts the fight she has her own question for (you).
For as long as she's known, she's been in Irkalla. Joy, pleasure, sadness, friendship; she's never had any of that, but she's had to watch her other half cartwheel around above, free as a bird this entire time. Are you really going to judge her? The person who's been working so hard all this time for no praise whatsoever?
You can say you don't think she's a bad person, but you're probably supposed to say you don't care since all of that was her duty. Gilgamesh concurs, as no duh no human would praise her for killing people.
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Gilgamesh says it's fine to lament about a duty forced upon you. It's also fine to abandon it entirely and seek another. But miring yourself in self-pity while doing it anyways is unacceptable. It's the same as insulting yourself. He finishes by agreeing with what you said.
If you pick the other option, Ereshkigal said she knew you'd understand and laughs in delight. She's certain she can take on anything now (which is why the fight is harder), and the fight starts immediately without Gilgamesh's interjection.
Incidentally, she's visibly hurt if you tell her you don't care. It's probably why the fight is easier.
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Once you crit Ereshkigal to death, Ishtar returns to her normal size and Gilgamesh regains his power. Meanwhile Ereshkigal is resigned to her fate. You can say you don't think she's a bad person again, and the fact she goes "you're saying that now?" makes me even more certain you were supposed to reject her self-pity session.
Anyway Ereshkigal sticks to her "I am the feared ruler of the underworld, I hate all of human life" shtick, even if you call her out for lying. So eventually robed guy comes out of nowhere and decides to take matters into his own hands. He slashes her, and then easily dodges the arrow Ishtar shoots at him. He tells your group to calm down, as he didn't kill Ereshkigal. He killed her fate. And indeed, she's fine, though she's a little freaked out since she thought she got cut in half. That might be a gag on the animation though.
Gilgamesh demands to know who robed guy is, since he's clearly no ordinary person. Neither Gilgamesh, Solomon, nor Chaldea summoned him, so who called him here? Robed guy just dodges the question and addresses Ereshkigal.
Robed guy says that Ereshkigal's lying, and that she actually loves humanity. You say you knew it, but Ereshkigal continues to claim she only likes the souls she owns and keeps shut up in Irkalla. You tell her that's wrong, and robed guy talks over Ereshkigal's protests and reveals that she doesn't like dead people. She likes humanity, to whom death is inevitable. He says a bunch of other stuff too, but hell if I can decipher his speech patterns. But the general gist is that Ereshkigal is a good goddess. Gilgamesh pipes in too about how while Ereshkigal may not be as powerful as Gorgon, she can put her forces together with Uruk's to make a formidable force.
Gilgamesh gives her one last chance, and turns to (you) to sway her over. You can either say you want to fight Gorgon alongside her, or that you've already been friends for a while now.
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Oh wait, a correction. Robed guy said he killed Ereshkigal's promise of alliance or something. I dunno, he talks weird.
Ereshkigal finally relents, but she has her pride so she says she'll repay your for this favor. If you're ever in need of help, call for her and she'll come bail you out. So with that said, she decides it's time to get back to administrating Irkalla so she gives you all permission to leave.
At that point, I think Gilgamesh asks Ishtar how you even get out of the afterlife anyways. Or something? All I really remember is that whatever answer Ishtar gives makes Gil bust out in laughter and makes you and Mashu go "sasuga Ishtar".
Anyway that's my summary of chapter 14. Jesus christ, I'm never summarizing a chapter without a text dump again.