Or a member of the Order of Odin having a showdown against some hoplites.
Grimms theory that Zisa could be the consort of the Norse god Tyr can easily point that way.
I'll post this here as link for people commenting:
Was there an actual Diedne covenant at Augsburg? I'm not too familiar with the House. If so, I guess that could be an explanation. That being said, if you want to use that piece of lore in your campaign as an inspiration, given that a number of historians seem to believe the initial source to be anachronistic and hardly credible, my first thought for an alternative explanation is a local faerie re-enacting a (probably deformed) story about roman difficulties in conquering the province of Raetia. I would not place the faerie and associated temple in Augsburg itself - Augsburg was founded as a Roman colony so that would make no sense, but probably in the general area. A genii loci could always be a tool if you have a strictly local divinity that is not mentioned anywhere else, but I like that explanation less than the faerie one given the details of what I can find on wikipedia about Zisa.