Minecraft, modloader and mod versions
Minecraft: 1.20.1
Fabric: 0.16.5
Soulslike-Weaponry: 1.1.5

Every time I kill the Harbinger of Moonlight (phase 2), he fully re-heals and keeps fighting. I've "killed" him 3 times.
After looking at the wiki, I can't find any specific methods needed to kill him.
After asking in the modpack's discord, someone said he must be killed at night, but this still didn't work. :(
It is also a commonly asked question within the discord that noone has been able to answer.

Other mods
Playing in Prominence II RPG Hasturian Era modpack.

This is odd, when it dies the first time it is supposed to change it's weapon from Nightfall to the Moonlight Greatsword, then when killed again it actually dies, it being night has no impact on it. If you quit out of the world, the state of the boss will naturally reset to the first phase again.

This is likely an issue with another mod or the server, since it doesn't happen on my end. Can you provide a video of it happening?

I would try to test the issue while removing mods one by one to see which one is causing the issue. Will close this due to inactivity soon if no response is given since this isn't a problem rooted in this mod, most likely another.

Thank you, will give the removing mods a go and report back!