Stigma is a ligature of the Greek letters sigma (S) and tau (Τ).
This monster appears to be floating above the black hole seen during Xyz Summons. As the Vylon were known as astrologists, this may indicate that Vylons Stigma, Pentacro, Tesseract and Disigma are from a different galaxy that the original Vylon have been searching for. This may also relate to how Pentacro and Tesseract are four-dimensional polygons, while the Vylon Tuners in the past were three-dimensional shapes.