So what is the Saberkeel style?

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The style from the Jubilee event https://eso-hub.com/en/fashion-outfits/saberkeel-panoply

Is there at least 1 NPC who wear this style in the game? What is this style in the lore? Who are wearing this, the bretons or the maormers?

It's sad to not have any information on this.

It look like a mix betwens maormers and breton knights... So again an other knight order who is not breton but this time maormer?

Eporem

I go here when I wish to know a little lore on certain styles that I have not yet collected..

https://en.uesp.net/wiki/Online:Styles

there it says this of the Saberkeel-Panoply style

The Saberkeel style is an outfit style themed after Nautical Knights.

if this helps..

Supreme_Atromancer

Yeah, I've made a similar observation in the past about the Snowhawk Mage style- Snowhawk existed as a city in Arena, but only as a ruined fort in TES5. Naturally, people who were paying attention were intrigued and wanted to know about who these "Snowhawk Mages" actually were, what their story was, what happened to the city. If it, like Saberkeel is just a low-effort thing with no story, I think that they could do a lot better, honestly.

I think in lieu of that, personally, I'd take it as a Maormer style- those upsweeping, fin/shell/wave-like motifs in the design gel very well with other "Maormer" styles, in particular Pyandonean: https://eso.mmo-fashion.com/category/database/miscellaneous/armor-weapons/style/pyandonean/, but also Dreadsails and Syrabanic Marine styles to an extent.