You’ve almost created Wildemount’s Lash of Shadows.
Explorer’s Guide to Wildemount officially introduced the Arms of the Betrayers as the weapon analog of Vestiges of Divergence. In comparison to these official items, your Ravel’s Fingernail is exceedingly similar to one of these Arms, the Lash of Shadows. But we need to do some fixing, the poisoning condition is much too strong.
The poison condition effect is probably too strong.
The top end of this weapon seems a little strong to me, but you will definitely find that having a chance to inflict poison every hit will prove much too powerful from levels 5-9 or so, and possibly through every level of play. To compare to the Arms of the Betrayers, the Lash of Shadows introduces a similar poison condition in its dormant state as a once per day feature, with unlimited uses of an extra damage feature. A chance to poison every hit is pretty strong for a weapon received at 15th level, but definitely too strong for earlier levels. I’d recommend something like adding one use per day per tier, so 1/day Dormant, 2/day Awakened, and 3/day Exalted. The progression mirrors the Lash of Shadows, at each stage it introduces a once per day poisoning feature, having three options available per day at Exalted level.
The Lash of Shadows is probably exactly what you’re looking for here.
As I’ve worked through the several revisions of this answer, my evaluation of your item has essentially turned into “you should just make this like the Lash of Shadows”. The balancing suggestions I’ve proposed in the previous section have turned your item into a slightly less interesting version of the Lash.
If you want an Artifact level weapon, just reskin the Lash of Shadows as a dagger. If you want a legendary level weapon, keep the +1, +2, +3 hit and damage progression, make the poison damage saving throw unlimited uses, and make the poison condition (1,2,3) per day based on its current state.
To be clear, the Lash of Shadows is still crazy strong. The unlimited uses poison damage effect is 3d6, with half on a successful save. If this is too strong for your taste, just scale it back a bit, or make it no damage on a successful save.
On Artifacts and “balance”.
As a final note, I would like to offer some commentary on the idea of “balance”. If you’re making an artifact, balance in a more traditional sense isn’t exactly what you’re looking for. The Dungeon Master’s Guide describes artifact as being tremendously powerful:
An artifact is a unique magic item of tremendous power, with its own origin and history. An artifact might have been created by gods or mortals of awesome power. It could have been created in the midst of a crisis that threatened a kingdom, a world, or the entire multiverse, and carry the weight of that pivotal moment in history.
Some artifacts appear when they are needed most. For others, the reverse is true; when discovered, the world trembles at the ramifications of the find. In either case, introducing an artifact into a campaign requires forethought. The artifact could be an item that opposing sides are hoping to claim, or it might be something the adventurers need to overcome their greatest challenge.
Introducing an artifact into your campaign will likely be inherently unbalanced, in the sense that you will have to present the players with significantly more difficult challenges than you would have otherwise. Balance is something of a relative term. If you give an artifact or three to the party without accounting for it in your encounter design, the artifacts will become an automatic win button, and things will got uninteresting quickly. So balance here is going to be about accounting for the power of the items in your encounter design.