Senmurv spawned on Brynhildr at roughly 3 AM ET.

Here is my FC's current theory about this S rank. We have triggered it 3 times immediately after completing the Fate "Cerf's Up." After completing it many times in a row, it reached its highest difficulty. With only two of us at the previous spawn of the Fate, it reset it its lowest difficulty and spawned much faster as well.

This idea coincides with fate farming on other servers. It would reach its highest difficulty then reset to lowest, thus spawning Senmurv.