TheShadowScout wrote: »

As far as I know, "Nirnhorned" is only available in Craglorn (and through crafting once your crafter learned it from those), and Agility/Endurance/Willpower is only available as vault reward in Imperial City. So the combination is pretty much impossibe to appear, right?

There is a small chance, but nirnhoned weapons and armor can appear randomly in various rewards outside of Craglorn, including dungeon drops, chests, VMA and various leaderboard rewards mails, including pvp campaign rewards.

Not sure if there is even any chance for it to drop in the IC vaults because I've never seen it. However, IC population dropped a lot prior to One Tamriel itemization update, so maybe there simply haven't been enough people opening the vaults?
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Reorx_Holybeard wrote: »

Willpower/Agility/Endurance items do not exist with the Nirnhoned trait in the mined game data we have so it's a safe bet to say its not possible. You can use our advanced item search to look for item combinations like this.

Wow, this is incredibly helpful, thanks!