For the campaign I'm running, one of the players had a fairly detailed request for an artifact weapon. I think I did an alright job but I'm too close to the situation to see things objectively and could really use so feedback/critique. I'm sorry in advance for the lengthy read.

Vajra the Demon blade
Weapon (longsword), artifact (required attunement)

(omitting backstory as this post is long enough, available upon request)

Deathseeker: Whenever Vajra is used to reduce a character to 0 hit points it has a 50% chance to kill the target even if nonlethal dmg was intended.

Soulflayer: Vajra’s powerful demonic presence and origin give it cruel powers beyond the physical or even magical properties normally found in the world. Upon being used to slay a creature with a soul (usually a humanoid) Vajra will attempt to absorb the soul into itself, forcing it into a pocket dimension identical to the Abyss. The wielder can either aid Vajra in this capture or attempt to prevent it. The wielder makes a DC 15 Wisdom check. Vajra chances or absorbing the soul change based off the nature and success of this chance Success with intention to aid: Vajra rolls a d4, on a 4 the soul is absorbed Failure: Vajra rolls a d6, on a 6 the soul is absorbed Success with intention to prevent: Vajra rolls a d12, on a 12 the soul is absorbed

Magic Weapon. Vajra the Demon Blade is a magic weapon that grants bonuses based on the number of souls it has absorbed.

  • 0: +1 longword
  • 1+: glows within 120ft of celestials, gains the thrown property (20/60)
  • 3+: +1 casting focus (+1 to spell attack rolls), as a bonus action can glow brightly causing bright light for 10 ft and dim light for 20 ft.
  • 5+: Vajra’s damage dice change from 1d8 (1d10 versatile) to 1d10 (1d12 versatile)
  • 7+: +2 longsword, thrown range is increased to (30/80)
  • 8+: +2 casting focus (+2 to spell attack rolls), gains the ability to cast true strike as a bonus action. The feature cannot be used again until after a long rest.
  • 10+: When Vajra gains 10 souls its demonic influence begins be able to affect the space around it. You can, as a bonus action, make Vajra change forms and appear as any bladed weapon. Additionally, it can hide its own magical aura, but any creature can make a DC 15 investigation check to see the magical aura. It cannot imitate other magical auras.
  • 12+: +1 AC, on hit deals an additional 1d4 lighting dmg and an additional 1d4 necrotic dmg. In addition, if Vajra is brought into the Astral Plane it is able to make attack rolls against a beings Astral Cord.
  • 15+: Upon gaining 15 souls Vajra full demonic might comes to bare. Vajra becomes a +3 longsword and a +3 casting focus (+3 to spell attack rolls). Vajra’s previous on hit dmg is replaced by 1d12 piercing (2d8 versatile), 1d6 lightning dmg, and 2d6 necrotic dmg.
    Submision: As long as Vajra’s wielder is holding it, he gains the ability to submit to Vajra’s influence OPD (one per day [long rest]) Vajra’s wielder can submit his will to the blades demonic influence. The wielders alignment immediately changes to CE, adds demon to its race, and automatically fails any charisma checks for the duration. The wielder gains advantage on all attacks using the blade and gains an extra action per turn that can only be used to cast vampiric touch as a 5th lvl spell. Once activated this change cannot be undone without a successful Wisdom saving throw DC 18 which can be attempted at the end of each turn.

closed as too broad by Purple Monkey, Miniman, SevenSidedDie Aug 30 '16 at 23:55

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    \$\begingroup\$ "a pocket dimension identical to the Abyss" - given that the Abyss is functionally infinite and contains a functionally infinite number of demons, do you mean that the pocket dimension looks like the Abyss? \$\endgroup\$ – Miniman Aug 30 '16 at 23:47
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    \$\begingroup\$ Also, balanced with respect to...what? Existing artifacts? \$\endgroup\$ – Miniman Aug 30 '16 at 23:48
  • \$\begingroup\$ @Miniman similar to the Abyss was my intention, ill change the txt. And yes im worried that im making this artifact either too strong or too weak relative to how artifacts should be. I have very little experience with higher lvl play and the DMG guidelines on artifacts are very helpful but im still concerned \$\endgroup\$ – ArtaSoral Aug 30 '16 at 23:56
  • \$\begingroup\$ We can't give general feedback and thoughts, because of the way this site works. For thoughts and feedback you'll want a discussion forum instead. \$\endgroup\$ – SevenSidedDie Aug 30 '16 at 23:59
  • \$\begingroup\$ I would like to offer comments about this question. I know we're not supposed to post answers in comments, but can I post a link to a chatroom? chat.stackexchange.com/rooms/44711/demon-sword-vajra-discussion \$\endgroup\$ – Dan B Aug 31 '16 at 0:38