This weapon is, without a doubt, extremely powerful, especially when compared with most items. However, it is a unique item in a campaign where I intend for all players to have at least one such artifact-level item; the rogue who winds up wielding this item shouldn't be too far out of line with the others.
For these items, I use the pattern from the Tal'dorei setting guide, in which players are rewarded for dramatic roleplay moments and leveling up by a deepened connection with the weapon-- which manifests as improved stats. Generally, the guidance is for the base level to be suitable around level 5, awakened at level 10, and exalted at level 15.
This weapon is intended to be the same one as is seen in Planescape: Torment, which is a video game, albeit an officially licensed D&D one. The stats of the dagger in the game are as follows:
Damage: 2-7 Piercing
Special: 2-12 Poison Damage
I will freely admit to not being familiar enough with the mechanics of AD&D 2e to have any idea of how these numbers shake out in the context of the tabletop game. However, given the bounded accuracy thing in 5e, I don't think it matters too much whether or not this dagger would make sense within 2e, since it would require a full rebalance to make sense in 5e in any case.
Here is my attempt at recreating this weapon for 5e:
Weapon(dagger), legendary(requires attunement)
This twisted fingernail of extruded night hag skin has got to be one of the ugliest things you have ever seen. Its mottled green, red, and black surface is almost a foot long, and it seems like it was used alternately as a scratching aid, nose-picking tool, and gardening trowel.
You have a +1 to damage and attack rolls made with this dagger. On a hit, the target must make a DC 13 Constitution saving throw. On a failure, the target takes 1d10 poison damage and is poisoned for the next minute.
When a character awakens Ravel’s Fingernail, apply the following changes to the item’s traits:
The dagger becomes a +2 dagger, instead of a +1 dagger.
The DC to save against the poison damage increases to 15.
When a character exalts Ravel’s Fingernail, apply the following changes to the item’s traits:
The dagger becomes a +3 dagger, instead of a +2 dagger.
The damage from the poison effect increases to 1d12.
The dagger winds up being one level higher than the original dagger, but this is because of the three-tiered system for this sort of artifact.
The poison is significantly more dangerous than the most similar weapon currently existing, the Dagger of Venom; it damages on every hit, rather than once per day, although the damage on Ravel's Fingernail is half of that from the Dagger of Venom. This is intentional. The Dagger of Venom is a rare item, and I would like this artifact to feel noticeably more powerful and cool than that.
Mostly, I'm concerned with the poison damage and poisoned condition on this weapon, but any other feedback regarding how balanced this weapon (remembering that it is an artifact weapon) is would be welcome.