Hey there Dante & Vergil!
...do like a bit of Devil May Cry myself.
Firstly thanks to Beefermatic for replying already. Cheers amigo.
I have an artifact, that wants to have the ability to utilize and/or summon previous users of said artifact that died wearing/using it, but I don't think that there is anything in the books that would enable this.
Sounds tangentially a bit like the Hand of Kwll from the Eternal Champion Corum series (a favourite of mine), reading the Elric series at the moment FWIW.
The Hand of Kwll is a supernatural artefact that was grafted onto the stump of Corum Jhaelen Irsei's missing left hand. It has the appearance of a six-fingered jeweled gauntlet of alien design. In reality, it is the actual severed hand of the Lost God Kwll, having been cut off eons before. The...
stormbringer.fandom.com
If I were to make a new ability for this artifact to use, what rank would be balanced for it?
I think the
first question to answer is who created the artifact, as that will determine the ceiling limit of the power.
In the case of the Hand of Kwll, Kwll would be a power beyond the Greater Gods. The Hand allowed Corum to summon those he had slain (seemingly on a per encounter basis in that if he killed a group of monsters in one fight he could summon the spirits of the whole group to aid him in the next fight). In this instance its unknown if the hand could 'contain' the power of beings greater than Kwll and that might be a moot point anyway given you would still need to defeat them in combat.
Your artifact allows the wielder to summon the spirits of the previous users. My suggestion as to how this operates is that summoning each past user extols a heavy penance upon the wearer.
For instance lets say the artifact summoned a great champion of lore (a powerful fighter). At the end of the encounter, your character is "weakened" for the next 3 (meaningful* ) encounters.
*as determined by the DM (so you can't cheat and just go off and face three groups of random goblins).
Whatever the summoned creature is good at, after the fight your character becomes weak towards - for 3 encounters. Make this encounter based, not time based so you cannot just wait out a time limit.
Hypothetically you could summon multiple spirits - but each enacts its own
personal toll upon the wielder's mind, body and soul.
As to how to balance the power of the summoned spirits. Firstly, make sure they siphon some share of the XP rewarded. If the summoned spirit is more powerful than the summoner, then consider them siphoning two or more shares of the XP as follows.
For instance if your character is Level 18 ( = CR 12,
NB. CR is 2/3rds Level ) and the summoned character is CR 16, then give them two shares of XP (+1 share). If they are CR 20 give them 3 shares (+2 shares), CR 24 give them 5 shares (+4 shares), CR 28 give them 9 shares (+8 shares), etc. Double the number of shares for every 4 points of CR above the wielder.
With this method, if the summoned spirits are doing all the 'heavy lifting' in the encounter, then they'll be getting the bulk of the XP from the group) and the wielder will be suffering the detrimental effects.
You might even add a small clause that the penance affects the whole group to a small extent (though not to the same extent as the wielder). Or that the wielder can share the penance among multiple characters to reduce the amount of encounters the effect lasts for.
Anyway, hope some of those ideas help.