Have not seen this before. A little over 2 cm.
It's a flat-back millipede in the order Polysesmida. At first, because of the relatively few number of body segments, I thought that it might be a pill millipede in the order Sphaerotheriida, but then realized that its antennae were not typical for that taxon. Unfortunately, millipedes as a group have not been all that well-studied from a taxonomic standpoint - seehttps://journals.plos.org/plosone/article?id=10.1371/journal.pone.0037240