SCP Foundation

The Brotherhood of Steel, which Veronica is a part of, is threatening to kill Veronica and you because she went against the Brotherhood code. You avoid the fight all-together by stating that Veronica only makes their ideals stronger by challenging their code, and to kill her for speaking against them would mean that they admit that their code might be wrong, since they would want to silence opposition.

They let you go.

The words that the Pale King uttered to the Vessels created to contain the Radiance (the light that plagues the Hallownest kingdom's dreams). In chronological order:

1. All the failed Vessels, who would later be thrown to the Wyrm pit to die and roam it as spirits.
2. The newest Vessel, the Hollow Knight, shortly before containing the Radiance.
3. You, a failed Vessel, going back to Hallownest and having severe flashbacks after seeing your kindred's corpses.

About the line itself, it explains the conditions a perfect Vessel must fulfill to effectively contain the Radiance, to be made by the Pale King (a.k.a. Wyrm, the being who overthrew the Radiance as chief deity) with the Void (a substance that transcends life, death, and time itself) and absolutely show no emotion, for the Radiance corrupts the slightest hint of one. It also shows that the Pale King will risk hundreds of souls just to stop the Radiance from corrupting the people of Hallownest.

After so many failed attempts, the Pale King finally succeeded in creating the perfect Vessel, the aforementioned Hollow Knight. Overjoyed, the King raised the Vessel like his own child, gently helping it to come to terms with its purpose in existence. The time soon came for the Hollow Knight to contain the Radiance once and for all.

It failed. The Hollow Knight was corrupted by the Radiance, the people of Hallownest started to go insane with every waking day, and the Pale King fell into despair and passed away in his throne, ruling a kingdom of corpses and madmen.

You soon came back (somehow) to Hallownest, and as you traverse the palace to find the source of the plague, you stumble upon a great chasm, filled to the brim with the bodies of the failed Vessels, their spirits roaming about. As you climb your way back out, the quote is repeated once again, all the while some good ass music plays in the background. Your memories start to resurface, and you see the last moments of your previous life, when you were cast down to die.

You hung on the edge of the chasm, seeing the King and the chosen Vessel about to walk through the gate leading back to the palace. The King rushed off without so much as a glance, but the Hollow Knight hesitated, looking back at you with what could be described as pity, sorrow, even sadness, as your grip loosened and you were swallowed up by the darkness below.

Following the trip on memory lane, you continue your track and make your way to the room used to seal the Radiance. You see the Hollow Knight, the last of your kin, towering over you, its eyes seething with the sickness it was supposed to contain. It arms its blade against you, and you do the same.

Now, if you took the Path of Pain (won't elaborate here because this is slowly turning to a thesis) and hit the corrupted Hollow Knight with the Dream Nail, you start to get a better sense of why it failed.

The Knight felt love. Love for Hallownest, for the people in there, for its fallen kin, and most of all, for its father, the one who brought it to this world. The Dream Nail dialogue mostly consists of insults and anger of the Radiance towards you, except for one exception:

Father?

You keep on fighting, attempting to reach to the source of the plague and, hopefully, fulfill the task that your "perfect" sibling failed at. The Knight knows this as well, as midway through the fight, it would sometimes just stop attacking and start stabbing itself, puncturing the organs plagued by the light. The fight nears its end, and no matter which of the 3 endings you get (5, if you count the Godseeker DLC), you still would have struck down the Knight. Whether you choose to contain the Radiance or find a solution to destroy it is up to you. No one left but yourself to realize, you are the perfect Vessel, the true Hollow Knight.