Jomsborg is a semi-legendary viking fotress, which may or may not have existed, perhaps on the shore, or an island just offshore of the Baltic coast of Poland.
Homebase of the elite "army" of Jomsvikings, it was supposedly destroyed in 1043 and sank beneath the waves.

In Mythice Europe, might it have been the target of Hermetic forces attempting to strike at the Order of Odin?


Sure, why not. Sounds like something a Flambeau Milites might do for sport or Tremeres for the ghosts of dead non-christian warriors.

Hermetic Magicians being so powerful, one expects to see in history some examples of their power being exercised.

I think the only canon example comes from 4th Ed "Heirs of Merlin", where there is a Peripheral Code ruling about the ritual creation of too much silver causing the value in circulation to halve in a decade.

In my Saga, the excess silver starts when Henry II starts mining extra lead to cover the roofs of all the churches he has to do as for penance regarding the murder of St Thomas Beckett. Silver is a byproduct of lead mining. This causes enough extra silver to enter the economy that it starts getting devalued.
Various Magi and Covenants suddenly find they don't have sufficient mundane funds, and within a year or two of each other, magically create silver. The minimum ritual to create silver creates a lot of silver. All these Magi and Covenants now flush with silver go on a spending spree. This causes the value of silver to drop precipitously. Then comes the Peripheral Code ruling.

I keep thinking there must be other historical events that Magi have influenced. Trying to find some.