Fate/Grand Order: Himuro no Tenchi - 7-nin no Saikyou Ijin-hen

This is a stand-alone non-canonical comedy OVA similar to Carnival Phantasm, you don't have to watch it in order to watch the Babylonia anime.
or understand grand order.

That said it is pretty fun if you are already familiar with the characters. It centers around the idea that a certain no good track and field trio from Fate/ Stay Night/ Hollow Ataraxia appears in Chaldea as servants.

There are some other joke servants appearing that was apparently the winning submissions of a "create your own servant" competition in Japan.

Story, art, and music is nothing to write home about but it is perfectly serviceable for a quick comedy ... OVA.