No, it's not a defective Dungeons & Dragons die. This special Milikeshi Rubber Eraser, by Japanese stationery manufacturer Kokuyo, is for people who take their erasing precision seriously.

"The Kokuyo Milikeshi is an innovative eraser with 5 edge sizes in one implement! Using just one eraser you can choose from erasing with a sharp corner, a 3 mm edge, a 4 mm edge, a 5 mm edge, or a 6 mm edge! The Milikeshi was designed by visual designer, Mr. Yuji Baba, Professor Emeritus of Tohoku University of Art and Design."

I suppose it is a creative alternative to eraser shields. I think in a parallel universe where eraser shields were never invented, all 20th-century draftspeople have these at their desks.