The route to its arena is a little confusing.
The Lampylumen Myriad boss is one of the many you can unlock in Wuthering Waves once you've been playing the game for a little while. Though it's slightly unclear what exactly this boss is—beyond just a giant frost monster—you'll need to beat it to acquire Sound-Keeping Tacet Cores to level your starting healer, Baizhi.
The problem is the boss is pretty tough to find. Just like Regisvine bosses in Genshin Impact, which also have a tendency to be concealed underground, you'll need to take a specific route if you want to find the boss arena and unlock a resonance beacon fast travel point so you can get back there quickly. With that in mind, here's where to find the Lampylumen Myriad.
First off, you'll need to reach Union Level 10 to unlock boss challenges. When you do, boss icons will appear on the map throughout the world, and you can challenge and claim rewards from them by spending Waveplates—effectively your daily energy limit. Most of these boss arenas aren't too hard to find, but the Lampylumen Myriad is a pain to locate.
You'll need to head to the Tiger's Maw Mine area to the south of Jinzhou. This is where the boss icon is, but while on the map it looks like it's on top of a mountain, the Lampylumen Myriad is actually underground, only accessible through the mine itself.
Here's the step-by-step of how to reach the arena:
The boss is relatively easy to beat provided you level a couple of characters to 40, equip some echoes, and bring a healer like Baizhi with you. Its trickiest attack is when it slams a claw into the floor three times, creating a frosty AoE shockwave with each strike. Luckily, you can dodge this the same as any other attack, so don't try to jump it—just hit the dodge button (shift on PC) as it hits the arena floor to dance out of the way.
Being a giant frost monster, it has increased resistance to glacio damage, so you'll also want to hurt it with non-glacio characters. Once beaten, you can claim its rewards from the centre of the arena for 60 waveplate, including that Sound-Keeping Tacet Core breakthrough material for levelling.