Arms of Relequen comes in 2 versions, an imperfect and a perfect version. The perfect version is slightly stronger in terms of stats than the imperfect version (additional Stamina bonus). The 5-piece bonus otherwise does the same thing on both versions.
Arms of Relequen is part of the Summerset Chapter. It drops in the Cloudrest Mini-Trial in Summerset. Each set item is bond on pickup. You can acquire medium armor, weapons, and jewelry of this set in the Welkynar motif style.