2 things i'd like to have an explanation for.

[Herr Gutentauben]

Why does the king's emblem become the void heart?

And why was the knight out in the cliffs in the beginning? Wasn't the birthplace sealed after the Pale King and the original HK left?

[Darc Vader]

I’m not sure if either of those has a definitive answer. As for the second one at least, I see three possible ways: there is another way out that we can’t access during gameplay due to the limitations of a 2d world; there was a second way out that collapsed at some point before we get down there; the king didn’t seal the Abyss immediately after taking the Hollow Knight from there, giving time for other vessels to leave.

You’re not the only vessel to leave/escape the Abyss. The broken vessel is one, you get the mothwing cloak from what appears to be the corpse of a vessel, and Nosk’s lair seems to be littered with dead vessels.

[L4z3r]

Where did all the vessels come from and why are they all dead in the abyss? I get most of the story, but I was never able to figure that one out. Also, was the Hollow Knight sent to the Black Egg to contain the infection?

[syndras]

The vessels were created by the Pale King in the birthplace, most of they died in the abyss because the Pale King killed them because they were not pure. Some of they managed to flee as Darc Vader said. The Hollow Knight was supposed to be pure, so he has been seeled inside the Temple of the Black Egg with the Radiance in the Hollow Knight to contain the infection. But it wasn't pure ( the look of the Hollow Knight at the Knight in the birthplace reminiscence) so the infection started to spread again. If you want you can watch some lore videos on youtube, SuperForge has made some and they are really good.

We cannot know everything because it is just lore, not a story. To make your own opinion you have to listen carefully to each npc and written things. I had an opinion close to SuperForge but much more melancholic. I thought the bugs were infected because they remember the past of Radiance's reign and praying for the return of this gold era.

[Scorpix]

Originally posted by syndras:

The vessels were created by the Pale King in the birthplace, most of they died in the abyss because the Pale King killed them because they were not pure. Some of they managed to flee as Darc Vader said. The Hollow Knight was supposed to be pure, so he has been seeled inside the Temple of the Black Egg with the Radiance in the Hollow Knight to contain the infection. But it wasn't pure ( the look of the Hollow Knight at the Knight in the birthplace reminiscence) so the infection started to spread again. If you want you can watch some lore videos on youtube, SuperForge has made some and they are really good.

We cannot know everything because it is just lore, not a story. To make your own opinion you have to listen carefully to each npc and written things. I had an opinion close to SuperForge but much more melancholic. I thought the bugs were infected because they remember the past of Radiance's reign and praying for the return of this gold era.

In my opinnion the vessel which was chosen by Pale King was pure, but as we can see at the end of the Path of Pain, everything has failed when Pale King started to feel something towards his perfect vessel and maybe threat him as his own son.

Our hero is not pure and the only chance to gain purenest it to venture into the Abyss and fuse himself with Hollow Knight memories to get Void Hearth.

Few vessels has escaped and probably that is why protagonist is still alive.

About Infection. I have found theory that it was created accidentally by The Radiance, but even though more important for her was to end the reings of Pale King than her peoples mental health.

[Eklipser]

About second part we simply don't know. There are speculations like those mentioned above.

But about first one: Why Kingsoul turns into Void Heart after visiting Birthplace, I think I have an explanation (at least a speculation like about anything in this game since nothing here is sure).

By collecting Kingsoul and progressing the game towards 3rd ending (Radiance fight) as vessel we discover / reveal our "story". How vessels were created (kinda) and what was their purpose. Half of Kingsoul we get from The White Lady, Pale King's wife (safe to assume), which was involved with vessels creation (possibly mother of all vessels?) and knows a lot about them and their purpose especialy about The Hollow Knight / "pure" vessel. She gives us one half of Kingsoul charm and with it her part of our story (if we are so kind to talk to her) and later hints about it when we come to her with two different but also the same charms equipped.

Second half we get from completing the White Palace, Pale King's residence hides some secrets about how vessels were created from liquid substance of the void. The creation of vessels and other constructs in order to do Wyrm's bidding. That's where we get second half of Kingsoul charm, the Pale King's "part" of our story.

When we visit the Birthplace we are still considered "different" by residing shades / our siblings here. Only after digging up the truth about the past and choosing of the "pure" vessel by Pale King we symbolically reconcile with our past and unite with siblings. We complete our "backstory" and accept that we are the creature of Void and Pale King's soul and shell. By revealing the truth and accepting it we reunite with voidkin and they cease to being hostile towards us. That's why Kingsoul charm (the past from Pale King and White Lady) becomes the Void Heart (acceptance of what we are now) [essencialy the void given form]. Symbolically moving on from past to now / uncovering the truth / accepting your being / transforming from Wyrm's creation following unspoken orders to void entity with its own purpose (except completing the game for gameplay's sake).
Or something like that. That's how I feel about it.