Radiance

The Radiance is the true final boss of Hollow Knight.

Physical Description
The Radiance has a feathery white body, crowned with a crest of three spikes. It has two glowing eyes shining from what seems to be a dark slit in its body, a pair of moth-like wings and a metallic-looking tail or legs. Its design resembles an angel and a moth.

In battle
In the first phase of the fight, the Radiance levitates slightly above the floor of its arena. Only its body is vulnerable to attack, and requires a jump to reach. It will often teleport around, making it even harder to get a good string of hits in while avoiding its own ruthless attacks.

The Radiance does not deal contact damage if the Knight happens to touch its body. However, all of its attacks (except hazards like spikes and the Void) deal two damage upon hit, making it by far the hardest-hitting boss in the game.

The Radiance has a variety of attacks: For the first phase of the fight, 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 is very important not to try to panic-heal despite the massive damage of its attacks, as any mistake in this fight is extremely costly. Wait for a good occasion to heal, such as during the third salvo of its eye beams.
 * It can shoot divine beams from its eyes, firing them three times in different directions.
 * It can fire eight swords from its body in a radial pattern. They gently curve as they fly.
 * It can rain down swords from the sky. There are always gaps in the waves to make it easier to avoid them, but it is possible to squeeze yourself between the swords even in thick clusters.
 * It can summon walls of swords horizontally, again with gaps in-between the swords. Sometimes this attack can be avoided simply by standing still if the gap is at ground level. The Shadow Dash can be used to pass through the swords unharmed, but the waves of swords come faster than its cooldown finishes, so in many cases you will need precise jumping to dash through the gaps.
 * It can fire a massive beam of light from the skies, sweeping the entire arena. You need to use a Shadow Dash to move through it, as it is otherwise unavoidable.
 * It can fire three homing golden orbs, one after the other. The orbs spawn in random places around the arena, similarly to the Soul Master's second desperation attack, and their homing factor is quite strong.

In the second phase of the fight, the Radiance continues doing the same attacks, but will now cover half of the arena in spikes. The restrained space makes it much harder to maneuver and to stall out the cooldown of the Shadow Dash for the sword walls. The Radiance will switch the location of the spikes every so often, outlining their new location with a golden glow for a few seconds as a warning before they actually emerge. Thankfully these spikes only deal one damage. The strategy remains mostly the same, but more precise dodging will often be necessary.

In the third phase, the Radiance will float in the center of the platform, place spikes at its edges and endlessly rain down swords from the sky. Since this is one of its easier attacks to avoid, this is mostly a breather phase.

In the next phase, the Radiance resumes using all of its normal attacks, but you are now fighting it on a field of small platforms over the Void. This makes all of the Radiance's 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 the Radiance will still teleport around, you will sometimes be able to hit it without jumping if it positions itself close to a platform. This makes it much easier to burst the Radiance down, though be wary of standing too close to it, especially if your health is low.

The last phase of the fight involves ascending by jumping up some more platforms as the Radiance flees you again. Its only attack in this phase is a typically poorly aimed, single laser beam. Unless you have little health left and no Soul to focus, this is usually a breather phase. Upon reaching the Radiance, hit it one more time to end the fight. You then get to dish out a very satisfying beatdown to it (just spam the attack button).

Story
The Radiance is the original ruler of Hallownest, connecting the inhabitants with a hive mind until the Pale King showed up and banished it. Over time, the Radiance was forgotten by the bugs of Hallownest, only a few remembered it, such as the Seer.

Vengeful over being forgotten, the Radiance infected the dreams of the residents of Hallownest, causing a terrible plague that made its victims stronger, but also drove them into a feral state, much like they used to be before the Pale King's arrival.

The Pale King attempted to keep the Radiance sealed by creating constructs of the Void, known as Vessels or Hollow Knights, to contain the plague. After many failed experiments, he thought he had succeeded, but in time, The Hollow Knight failed to keep the plague in check. He succumbed to the infection, which spread across Hallownest and led to its downfall.

Some time later, another Hollow Knight, with the help of Hornet, was able to use the Dream Nail to reach the Radiance directly inside the previous Hollow Knight. He defeated the mad deity by dragging it into the Void at the price of his own life, putting an end to the Radiance and its plague once and for all.

Trivia

 * In Japanese culture, moths are seen as symbols of death, and white as a color of death as well.