Radiance

Behaviour and Tactics
The Radiance is the true final boss of Hollow Knight. To access her, the player must have acquired Void Heart and have used the Dream Nail on the Hollow Knight when Hornet holds them down to be able to challenge her.

In the first phase of the fight, she levitates slightly above the floor. Only her body is vulnerable to attack, which requires jumping or ranged attacks. She will often teleport around, making it even harder to get a good string of hits in while avoiding her own ruthless attacks.

The Radiance does not deal contact damage, however, all of her attacks (except hazards like spikes and the Void) deal two masks damage upon hit, making her by far the hardest-hitting boss in the game.

As with most boss fights, Quick Focus is highly recommended. Since occasions to hit the Radiance are not too frequent and her attacks deal a lot of damage, SOUL-generation charms like Soul Catcher and Grubsong are also suggested. Fragile Strength can be a lifesaver by increasing damage output tremendously (to prevent the Charm from breaking, simply pause the game and reload before dying).

It should be noted that in the first three phases, the swordbeams from Grubberfly's Elegy can be used to hit The Radiance without jumping, which can be especially useful when combined with Quick Slash.

Joni's Blessing and Lifeblood Heart/Core can be useful for those finding it hard to heal.

A spell-focused build can be employed by forgoing any healing Charms for the Lifeblood charms described above along with Shaman Stone, allowing the player to spam Abyss Shriek to get quick and easy damage off, as the Radiance does not get knocked back and will take all four hits of the Spell.

In Phase 1, The Radiance will simply use these attacks while hovering above the floor and teleporting. The hardest attacks to avoid are typically the horizontal walls of swords and the homing projectiles. It's very important not to try to panic-heal, despite the massive damage of her attacks, as any mistake in this fight is extremely costly. Wait for a good occasion to heal, such as during the third salvo of the eye beams.

In Phase 2, The Radiance continues using the same attacks, but will now create spikes across alternating halves of the platform. The restrained space makes it much harder to maneuver and to stall out the cooldown of the Shadow Dash. She will switch the location of the spikes every 5 seconds, outlining their new location with a golden glow for about 2 seconds as a warning before they actually emerge. These spikes only deal one mask of damage. The strategy remains mostly the same, but more precise dodging will often be necessary.

In Phase 3, The Radiance will float in the center of the platform, place spikes at the edges and endlessly rain down swords from above. Since this is one of the easier attacks to avoid, this acts mostly as a breather phase.

In Phase 4, The Radiance resumes using all of her normal attacks, but she is now fought on a field of small platforms over the Void. This makes all of her attacks much harder to dodge without accidentally falling down into the Void and taking damage; put the double jump to good use. On the bright side, while she will still teleport around, it is still possible to hit her without jumping if she teleports close to a platform.

The last phase, Phase 5, involves ascending by jumping up platforms as The Radiance flees again. Her only attack in this phase is the Big Beam. Upon reaching the Radiance, hit her one more time with the Nail to end the fight and trigger the final sequence.

In the final sequence, The Radiance is bound by Void Tendrils and the shade of the Hollow Knight appears, tearing open a weak point on her face. The Knight bursts out of their body and becomes their Shade. Attacking causes the Shade to repeatedly whip The Radiance's weak point with long tendrils until it's defeated and dragged into the Void below.

Trivia
"The moth tribe were (perhaps) descended from Radiance. However, the King convinced them somehow to seal Radiance away. I guess so he could rule Hallownest with his singular vision, as a monarch/god with no other gods.The moths sealed Radiance away by forgetting about her. Hallownest was born and flourished. However, the memory of Radiance lingered (eg the statue at hallownest's crown) and soon she began to reappear in dreams and starting exerting influence. The King and the bugs of hallownest resisted this memory/power and it started to manifest as the infection. Thus the first attempt to seal Radiance failed, and the King had to try another method - the Vessel." Сияние
 * Beating the Radiance is not required to complete the Hunter's Journal, but she does have an entry.
 * Through HUD manipulation, The Radiance is shown to actually have 3000 health, though the "fight" portion of the battle ends when she reaches 1300 health, effectively requiring the player to only have to deal 1700 damage to win the fight.
 * Along with Nightmare King Grimm, The Radiance is the only boss that has a fullscreen title card.
 * The Radiance is confirmed to be female from developer commentary on Slack, as well as from developer notes regarding the Seer:
 * The Radiance is one of the two bosses that must be challenged to battle, the other being the Mantis Lords. While the Mantis Lords will have to be challenged every time, the Radiance's intro is abridged after the first time fighting it, causing her to attack directly upon entering her arena.