There are 7 schools of magic in dnd, so if we don't exclude necromancy we can have an extremely powerful artifact for each school. I think they could be balanced by having maybe few charges, or having difficult activation requirements, like specific places and times. One other option that comes to mind would be items that become more powerful the more powerful is the wielder, so they would seem and be remembered as astonishingly powerful, but were so because the mages were using them, and instead in the hands of the pcs are just a bit better than their nonlegendary counterpart

For scaling, I recommend having the items get more powerful as you find more of them. That way, the artifacts they find at levels 3-4 are still relevant at level 12. Maybe if the players pass an investigation/arcana check, they could use their existing artifacts to help determine when they're in the proximity of a new one.

So if you have an orb that is full of swirling clouds, it could let them cast Stinking Cloud at DC 12 at level 3 once per day. Then as more items are discovered, the DC would increase, and eventually it would cast Cloudkill with a DC of 17.

Other artifact ideas:

• a long, thin strip of leather (characters with a background in smithing or proficient in leatherwork recognize it as a handle grip). When wrapped around the handle of an ordinary weapon, it becomes a +1 magical weapon (upgrading to +2 and +3 with more artifacts).

• a gemstone that seems to absorb light. It allows the bearer to cast Counterspell once per day (with more artifacts, the Counterspell is cast at a higher level, and the gem gains additional powers, like being able to create an anti-magic sphere once per day).

• a figurine of a hammer. Once per day, it can be used to instantly repair a broken Tiny item (with more artifacts, the size of the item can be Small, then Medium, then Large, etc)

• a burnished shield that looks like it's never been used. It grants an additional +2 to AC (+4 total), and once per day, it can absorb all damage for a turn (with more artifacts, the ability protects others around you within an increasing radius)

• a charred staff. The bearer can cast Scorching Ray once per day (with more artifacts, it casts Fireball, Wall of Fire, Firestorm, etc once per day as well)

Do all of the items need to be directly useful to the players is a roleplay/combat situation? If not, you can make objectively powerful items with interesting effects with short durations, long cooldowns, or effects that scale as the wielder becomes more powerful. You can also give the items drawbacks until their secrets are uncovered.

An umbrella that can cause a light rain, a downpour, or a thunderstorm in a 10x10 mile area with a 1 week cooldown.

An orb/weapon that doubles dice damage for a single combat and causes two levels of exhaustion after the combat ends. Reduce to 1 lvl of exhaustion for characters with proficiency bonuses of +4 or higher.

A shield that summons a spectral guardian. The guardian cannot attack. On each of its turns, the guardian taunts all enemies within 10 feet. Those enemies make a DC 15 wisdom save. On a fail, attacks against targets other than the guardian have disadvantage until the end of the guardian's next turn. Cooldown: 1 day. Scale the DC of the save with quests to uncover more information about the item, or give it synergies with some of the other items which increases the DC when the shield is within 120 feet of the umbrella.

You can also make the items deteriorate with each use until their secrets are fully uncovered.

A skeleton key which unlocks any lock it encounters. After each use, more and more cracks appear. After the third use, roll a d20. On a 1 or 2, the key's magic fails and explodes dealing 8d6 damage in a 20-foot radius. You wouldn't reveal the full consequences of the item, but you could describe the item getting hot and showing visible damage. Giv the players arcana checks to try to figure out what's going on.

The possibilities are endless, and you make the rules. The items don't need to be 100% useful when they're discovered, and they can get more powerful so the players want to keep using them.

In the campaign I'm running, there are 7 pieces of an artifact that do something on their own, but can be combined to make a much more powerful artifact. Each relate to an inner plane - earth, fire, water, air, negative, positive, prime. Each has charges and allow you to do different things, e.g. the negative material plane piece allows you to use Vampiric Touch.

Luck swords are great narrative builders, basically a sword with a rechargeable wish. Lets you do some fun things with the world building before the party shows up.

I mean, without detail, all you can get is vague suggestions.

With that in mind, I would look into the d20srd "paragon" (I think that's the name of it) mini-prestige class. Have each item grant some (or all) of those benefits. https://www.d20srd.org/srd/variant/races/racialParagonClasses.htm

I've used them a few times here and there for flexible power boosts in one way or another. As prestige classes go, they're kinda weak, but worth considering. But, as a kind of guide to what powers exemplify a race, they're golden.

If your base an artifact off of one, you've got what amounts to a pretty nice boost at any level, but not one that's game breaking.

As an example, have each artifact give the level bump effect you want from the attack bonus and saves, then just tack on the special abilities as whole, or just the ones you want, until you reach the kind of power and flavor you want.

They aren't all equally useful, tbh, but they are all useful.

Since you want historical import, such a council of mages may represent the first attempt to really have cross-species cooperation and understanding. I can't think of anything better for that than a ring of power that literally gives you access to what a race experiences.

If you want to tone things down a little, you could just pull whatever of the racial abilities you want rather than the enhanced versions from the class, but I don't see anything game breaking in there.

It may seem under artifact power levels, but they'd be so broad in scope that I think it would fit.

Besides, powering them up is just as easy as toning them down. Give them the ability to have some kind of advantage when dealing with the race that a specific ring represents, maybe the ability to speak/read their language (where appropriate), and the ability to polymorph into that race at a given level of frequency.

I have a similar set of items in my setting, but I really enjoy the juxtaposition of mundane items doing really unique and powerful things. One of them is an old rotten tooth that can slowly destroys organic material. One is a shaving razor that allows you to cut portals over short distances, perfect for rogues taking coin purses. Some of the other items include a comb, a chess piece, a broach, and a coin. I think the best way to balance powerful items is in the times they can use them per day. If it's a crazy strong ability, maybe it takes a week to charge up for one use. If you feel it isn't very useful, let them use it 3/day and see if they surprise you with some creative thinking. Tweaking those time-frames can make even something initially unimpressive feel like a true relic.