It's a cool shield and Cthulhu is mostly public domain but I suspect it's the very blatant copying of Mindflayers from Dungeons & Dragons that would make SE wary of even publishing this image.
My design "Cthulu's Honor" was not listed among the entries this year for unknown reasons and I never received any notification to clarify (assuming the name Cthulu is copyright / trademark protected) . So i decided to submit it here.