You continue doing a marvelous job of building tension while Tristitia ventures out into the town, seeing the physical aftermath of the attacks, telling us it wasn't just a spirit attack, it wasn't just in her imagination, there was actual destruction, meaning there's an actual threat. The fact that the heartbeat key is out of sync with her own is a perfect little detail, as well. The rest of my comments are spoilery, so in case someone is reading this before the chapter, I'll wrap them up. You, hypothetical person doing that, go read!
I don't recall if the elder told her where to find the bunker, but if the fountain is one of the oldest things in the village, I could easily see her finding it. It brings up a question, though: how did the invaders have the key, and how did they get into the bunker while the elder held one and Tristitia the other? I suppose they could have gone for the elder first to get his key after losing theirs, but... how would they know who had it?
I love the entire scene inside the bunker, even though I hate what happens to these characters we're just beginning to grow close to. Seeing the heartbeat match up to the key immediately told me what was going to happen, a perfect example of subtle but effective foreshadowing. This is a perfect visualization of the power struggle, and one happening in the worst possible place. Tristitia's internal battle is fraught and desperate, making it even more heartbreaking when it's not nearly fast enough. The story comes full circle, beginning with Tristitia feeling isolated and ending with her actually being isolated. It's a shocking conclusion and melancholy, and the key matching beat with her heart punctuates it amazingly.
But... this can't be just a 4-parter.
The mystery of Libra hasn't been resolved. Initially I was thinking she must be the vistra, but no, Laetitia sees the monster before Libra leaves the house, and while a naga might be frightening to see, Laetitia would know what to call them. I had thought Libra's body or a facsimile might be controlled by the naga, but no, she seems to have at least some memories of the real Libra. The angle of the rogue naga trying to extend the war is intriguing, but there remain questions: why Tacet Petram? How did they know of the bunker? How did they capture and control the vistra so that it didn't kill the villagers? The very first thing it did upon transferring to Tristitia was go on a murderous rampage.
Finally, what happened to Libra? I'm certain now that she wasn't the creature, I'm pretty confident that she wasn't just an illusion, and she's too important to the story to have her (re)death glossed over without mention.
This dish is still cold in the center, it's not done, yet!