6th Ending: Go to Darkness

This is obtained by beating the Pantheon of the Vessel after completing all of The Grand Trials

What happens
After beating the Firstborn, it becomes a Shade and begins to attack the Knight. Then the walls get destroyed(ALL OF THEM) and the Radiance appears. The Knight, with no floor to stand on, falls into the abyss, revealing the Void Entity. It absorbs the Firstborn's Shade, creating the shade of what looks like a baby/chibi version of it, about the size of a crawlid. It then slashes the Radiance with all four of its claws, destroying her in an instant. It then picks up the knight, before crushing it, turning it to a shade.

It then cuts to the Lake of Unn, where all the Mosskin are looking at the lake, now full of void. Unn, also made of void, pops out, spreading the Void Entity's arms around, turning the Mosskin to black bushes with blue eyes, similar to Mosscreeps.

It then cuts to the Nightmare's Heart, where it will begin to Cry Void then more arms burst out, trying to grab the Grimmkin who are now running away.

It again cuts to the White Lady, who begins to become void. Her bindings then disintegrates, unleashing a blizzard of seeds, presumed to be new vessels.

It then cuts to the Pale King's Real Corpse, lying in bed. He wakes up and begins shrieking, as he is turning into a creature made of Void.

It then cuts to The Knight's real body, still lying by the Godseeker asleep. It then bursts with the arms which end up grabbing the Godseeker and pull her into the Knight, who becomes a shade with 4 horns similar to the void entity. The shade grows a second set of arms and eyes, as well as growing larger.

It then cuts to Hornet, who is walking out of the Temple of The Black egg with the Hollow Knight, no longer infected. Then a black object appears and the two of them get into battle stance.

Then the cutscene ends