ENDER LILIES: Quietus of the Knights

To get Ending C, first you must complete, then wear the Luminant Aegis Curio into the final boss fight. This guide should help you unlock that: https://www.neoseeker.com/ender-lilies-quietus-of-the-knight... The amulet itself costs 0 relic slots, so you should have free reign to use your preferred relics as normal and still have it equipped.

First things first, if you haven't seen Ending B, I highly suggest you do that first.
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For Ending C, you will be fighting the same boss again. However, (at least) 2 more phases and a second healthbar are added.

First, the boss will be fought as normal through the first 3 phases.

After defeating the boss in that first half of the fight, a short cutscene will play, the boss will take on a powered up glowing form, gain a new healthbar, and phase 4 will begin. As far as I could tell, with one exception, phase 4 is identical to phase 3, including in damage numbers, only the boss is all glowy now.

That one exception is a big one though, a new meteor storm attack. This attack potentially can be difficult to dodge at times, but doesn't quite reach the levels of a bullet hell shooter. There will be about 4 or so waves of large fireballs fired from the top middle of the screen that you will need to find gaps in. The last 2 waves will consist of fewer of the fireballs, but do be warned that those last few do have a slight homing capability that can make them a bit more difficult to dodge. In terms of damage, they still hurt certainly, but are a little bit less damaging than the other damage sources during this fight. Getting hit by multiples would almost certainly kill you outright though, so don't think you can just tank the damage.

The final phase (phase 5) I can confirm will be entered right before the boss dies. The boss will actually depower and lose its glowing form, turning a grey color. As far as I can tell, it loses the meteor attack, however, I cannot be certain of this as it died after only about 3 attacks after entering this form.

There may have been a phase changed in between the 2 of those. One of the phase change animations/blasts did happen during phase 4 before entering phase 5, but I never noticed anything different before and after that blast until entering the depowered phase.