The ESO arena set category contains all arena sets that drop from various places in ESO such as the solo Maelstrom & Vateshran Hollows Arena and the 4 player Blackrose Prison & Dragonstar Arena. The different arenas both offer unique ability altering weapon sets for ESO as well as normal 5 piece sets.
You can learn more about each arena here:
Special ability altering weapon sets drop in each arena in ESO. If you clear an arena in normal difficulty, you will obtain imperfect ability altering weapons, which are slightly weaker than the perfected version. If you clear an arena in veteran difficulty, you will obtain perfected ability altering weapons, which do have one extra bonus attached to it. However, the unique ability altering effect is the same on both version.