Thats the same exact identic symbol that Kouichi from The titan's bride manga had when Caius 'knocked him'- let it out inside of his butt****
Comment by Yaoi mmmnnn on Reincarnated into Demon King Evelogia's World - Chapter 32
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It's in other mangas/manhwas/whatever you like to read, usually called a 'Womb Mark' or 'Womb tattoo'