We are told (Ben Ish Chai year 2 miketz, and others...) that Mashiach ben Yosef will fall in the hands of Armilus HaRasha. Some poskim (see ילקוט יוסף תפילה ב הערות נוסחאות התפלה והברכות) who even say we need to pray in our Amidah that MBY will not die at the hands of Armilus. I even saw in a footnote (of מושגים תלמודיים גוג ומגוג) that Gog is Armilus.
Do we know anything else about this seemingly important (albeit evil) character? Please bring sources!