Given that we're talking about jp metal stuff, here's a dump of lesser known metal bands from jp i found over the years:
黒夜葬, Galneryus, Imperial Circus Dead Decadence, 5150, Denkare, Yousei Teikoku, Powerless/MuryokuP, UtsuP, Nocturnal Bloodlust, Matutinum Miserabilis, UI-70, まぐなむお~ぱす, Onmyo-za, Pearls Before Swine, Pizuya's Cell, Dragon Guardian, Yakui the Maid, Takamachi Walk, IRON ATTACK!, UNDEAD CORPORATION, i-chan, Unlucky Morpheus, Piece Of Rain, Vanitas Vanitatum, Caz, Diabolic Phantasma, Hone Your Sense, S.S.H
I tried to only put metal bands here, but some non-metal ones might have slipped by, also most of those are really hard to find. there are some pretty known ones tho. Some vocaloid stuff, some touhou stuff, some dead bands, sorted in no particular order, but most have at least something interesting to listen to.
If you like your metal a bit more on the extreme side Japan has you covered too.
Sete star sept, Goninishu, Gallhammer, Sigh, Abigail, Flagitious Idiosyncrasy in the Dilapidation*, Gotsu Totsu Kotsu, Transgressor, Zombie Ritual, Defiled, Sarugutuwa, Birushana, Heterogeneous Andead.
Just to name a few.