Might & Magic VI is strange in that you re-invent your entire equipment several times over the course of the game. At the beginning of the game, you want items that give you more hit and spell points because those allow you to stay alive longer and cast more spells during long fights. Minor enchantment that improves your stats by five to ten are good too because early on that often means an improvement of +2 or +3 to your stats. But then you teach your characters bodybuilding and meditation and those extra hit and spell points from your equipment feels like a waste of equipment slot. Then some of your stats reaches the fifty breakpoint*, and from this moment on, you need an improvement of twenty five to reach the next breakpoint and get the +1, and it seems like a waste to use three or four equipment slots to get a +1 bonus to a particular stat when you could use those slots to improve other stats. And then your buff spells improve all of your stats past the fifty breakpoint, and so you change most of your equipment to get more elemental resistance now that monsters starts using magic or elemental damage more often. And then the game start dropping artifacts and relics which are just too good to pass up, and then you get the Rings of X Magic and everything changes.
* For more info on breakpoints:
https://sites.google.com/site/sergroj/mm/mechanics In my most recent game, my cleric and sorcerer had 12 in Body, Spirit, Mind, Fire, Water, Air, Earth magic, to cover my bases, and I dumped everything else in Light and Dark magic. By the end of the game they both had 36 in their respective Light and Dark magic. The Guinevere artifact improved that by 50%, so they were now at 54. The Rings of Light/Dark improved those by another 50% of the newly modified stat (meaning 50% of 54, not 50% of 36), 54 + 27, they were not at 81 in Light and Dark magic. Add to that the followers that improve all your magic skills by four and three respectively, and they were now at 88 Light and Dark magic.
With 88 in Master Dark magic, when you cast Day of Protection, all your resistance improves by 352. With 88 in Master Light magic, after casting Day of the Gods and Hour of Power, all my stats were ranging between the 400 and 500. At that point I sold all the equipment that improved my resistance and most of my equipment that improved my stats, with the exception of some that allowed me to easily reach the 450 or 500 breakpoints on some specific stats. From now on I wanted equipment and artifacts that gave me hit recovery, lot AC (I was lucky enough to get Zeus before reaching my artifact limit) but more than anything else, I wanted equipment that gave me lot of hit points to tank those spells and dragons' breath attacks, lot of hit points for my Knight so I could spam the Shared Life spell, and lot spell points for my sorcerer to spam Dragonbreath and Shrapmetal during long fights. In that regard, equipment that gave +5 Levels were a nice additions on my knight and sorcerer to get more hit and spell points.
So that necklace you got, it of course heavily depends on how you leveled your sorcerer. If you intend to use many of the spells from the Fire, Air, Water and Earth schools of magic during the end of the game, you most likely will want to equip Morgan instead of Hera, but if you went all the way into Dark and Light magic, by the end of the game you will most assuredly be grateful that you held onto it.
And you probably want to add a second copy of Guinevere, and maybe Igraine and Morgan to your shopping list depending on your party composition as you can get duplicate of those. I was lucky enough to get one extra Igraine for my Paladin, and one extra Guinevere for my Cleric in addition to the three guaranteed drops from the obelisk treasure. I don't remember if I got them before or after opening the obelisk treasure tho, but considering I was also unlucky enough to get duplicates of Mordred and Excalibur after acquiring those from the quest and the rock, I'm tempted to say it doesn't really matter. I could be wrong tho.