นักมวยใต้ดิน

Missing, extremely new, or very obvious to obtain.

Underground fighting refers to unsanctioned, often illegal combat sports that take place outside of official regulations and governing bodies. It involves various martial arts, bare-knuckle boxing, street fighting, or even weaponized combat. It's also taken quite literally in the context of Star Rail, taking an enemy out under the bridge (ground) using the Hand of Zagreus' Punch Action.