Maintenance for the week of October 5:
• [COMPLETE] ESO Store and Account System for maintenance – October 8, 8:00AM EDT (12:00 UTC) – 12:00PM EDT (16:00 UTC)
From that site "Note: 5 item bonus was added in Patch V1.1.2". So, maybe all the website, like that one, are still "outdated" and maybe there truly is a new 5th piece somewhere. I think I'll post a bug report anyway.
I've been looking for new pieces to existing sets they've updated with new bonuses. Since the Craglorn patch, the only new set piece I've seen is the Clasp of the Dragon (Necklace). It's possible they are intending to add more pieces but haven't yet. That's my theory anyway.
I am thinking the Necromancer set is the same way. Originally started out as a three piece set. Now is a five piece set. Have only ever seen four pieces (Chest/Head/Shoulder/Leg). Have no idea if a fifth piece even exists. I keep looking on the stores and in zone chat. No luck.
I'm pretty sure there are only 4 pieces at the moment. Yesterday I spent a good hour looking for a fifth piece, but all my five guild stores listed the same 4 pieces (chest, legs, shoulders and belt).
Guess thats a bug then, which makes this set rather useless.