the Mindflayer monster concept is owned by Wizards of the Coast
Mindflayer is owned by Wizards of the Coast — you cannot legally reuse it in games and other content without permission from them (at least under US copyright law).
Consider alternatives to non-free game monsters, from this site consisting only of open source DnD content:
https://www.5esrd.com/gamemastering/monsters-foes/ogc-replacements-for-classic-monsters/