The weirdest thing about this manga...is how it hasn't got a C&D notice from Hasbro (who owns Wizards of the Coast who owns D&D).
It is possible to use actual D&D mechanics in a work through an open license agreement, but mind flayers, especially the term illithid, is not in the IP package that WotC open licenses, and the terms are trademarked (at least in the US).
Japan doesn't have fair use in their copyright and no nominative use for trademarks, which is why brand names in things have to bland-inated; e.g. beholders in Dragon's Crown were instead "gazers".