I have a character that recently became an avatar of death for the Shadowfell, not the edge-lord kind, but the chill kind. The character is a halfling, multiclass, swordsmith face that is CG, and is styled as a carefree friend to everyone, even the villains sometimes to the party's disapproval. He truly believes that everyone is redeemable. The GM let me design a sword based on the sword of Zariel and looking at things like Blackrazor, Wave, and other legendary swords for power level. I used a few spell effect riders and class features that felt appropriate for a shadow/death feel, and the DM added the 3d8 damage. I envisioned the sword as an item that eats shadow and refuses the living to cheat death. We are level 17 and heading into the outer planes to battle whatever the heck hangs out there. Any advice or balance issues you see would be appreciated and yes, the others in my group have OP items as well.

(Rapier +3)
- Sentience: (TN) hearing and darkvision 120 feet. Telepathically communicates with wielder.
- INT18 (+4)  WIS20 (+5)  CHR10 (0)
- +3d8 necrotic damage
- Shadowfell Resistance: necrotic and cold
- Shadow Walk (BA): 120’ teleport from dim light or darkness to dim light or darkness.
- Aura of Certain Death: 10’ radius selected targets can't regain hit points
- Biting Darkness: 10’ radius light reduces by 1 level, shadows lash out for 2d8 necrotic whenever you are hit
- Raise Dead, Bestow Curse (5th), and Find Steed 1/day

the others in my group have OP items as well.

Then our opinion on balance is rather irrelevant because good balance for the game in general doesn't matter in a highly HBed game. What matters is balance with respect to the other players.

Good point. Maybe I was thinking about other features that would seem more in line with a Shadowfell god of death and the grave that I may have not thought of. Or a feature I added that does not seem to fit the idea.

Quote from theHumanChris >>

I have a character that recently became an avatar of death for the Shadowfell, not the edge-lord kind, but the chill kind. The character is a halfling, multiclass, swordsmith face that is CG, and is styled as a carefree friend to everyone, even the villains sometimes to the party's disapproval. He truly believes that everyone is redeemable. The GM let me design a sword based on the sword of Zariel and looking at things like Blackrazor, Wave, and other legendary swords for power level. I used a few spell effect riders and class features that felt appropriate for a shadow/death feel, and the DM added the 3d8 damage. I envisioned the sword as an item that eats shadow and refuses the living to cheat death. We are level 17 and heading into the outer planes to battle whatever the heck hangs out there. Any advice or balance issues you see would be appreciated and yes, the others in my group have OP items as well.

(Rapier +3)
- Sentience: (TN) hearing and darkvision 120 feet. Telepathically communicates with wielder.
- INT18 (+4)  WIS20 (+5)  CHR10 (0)
- +3d8 necrotic damage
- Shadowfell Resistance: necrotic and cold
- Shadow Walk (BA): 120’ teleport from dim light or darkness to dim light or darkness.
- Aura of Certain Death: 10’ radius selected targets can't regain hit points
- Biting Darkness: 10’ radius light reduces by 1 level, shadows lash out for 2d8 necrotic whenever you are hit
- Raise Dead, Bestow Curse (5th), and Find Steed 1/day

 Sentience seems fine.

Necrotic damage seems fine

Resistance seems fine. Maybe have one of them tied to "while in darkness"?

Shadow Walk seems out of place tbh.  Seems odd and abusable with a huge teleport distance for only a bonus action; because you're almost always going to be in dim light because of your Biting Darkness you basically have a free no-hit ability at all times, which also defeats the purpose of the Biting Darkness second ability.

Aura of Certain Death I would reduce to creatures within 5 feet, or 10 feet while in total darkness?

Biting Darkness is fine, but should have a range limit on the shadow lash, as in, "when a foe hits you with a melee attack while within 5 feet of you..." or something similar.

EDIT: These are just suggestions to provoke thought, not in any way meant to adjust it as a, "do this or it sucks!" ordeal, lol. As agile said, tough call to consider what is or isn't OP for that game.

Thank you Styles Striker. I was looking for play style tweaking because this is the first time I have ever been in a game that has gone this high level. I usually do not focus as much on damage as the rider effects. I chopped the Aura by expanding the  Spirit Shroud spells rider and Biting Darkness from the Shadow of Moil rider. I thought having a god of the Shadowfell moving through shadows sounded fitting and I looked at the shadow magic sorcerer ability. I agree 120' does feel too far of a move considering my character has 9 levels of swashbuckler and can BA dash.