What is the rarity of my homebrew shortbow?

I want to create a custom magic item - a shortbow in particular. I want it to have a illusion theme but also a psychic damage theme, so here's what I've come up with.

It will require attunement; it will grant the minor illusion cantrip (PHB, pg. 260) and it will do +2d4 psychic damage on hit, and on crit, it casts a high level illusion spell; I was thinking mental prison (XGtE, pg. 161), since it fits both the illusion and the psychic damage themes.

The spell save DC for mental prison I was going to set to 15, on par with other very rare magic items, and I was also going to limit this effect to once per day so that a string of lucky rolls doesn't reign down 6th level spells on everyone (I will allow the wielder the option of using this or not on a crit, similar to how a paladin can choose to use Divine Smite or not on hit).

Note that I would consider the damage that the spell does to be secondary damage and therefore not subject to being doubled due to the attack being a critical hit (see these two questions for more detail on that).

(Name to be decided)

Weapon (shortbow), ??? (requires attunement)

While you are attuned to this weapon, you know the minor illusion cantrip and can cast it at will. The weapon deals an extra 2d4 psychic damage to any target it hits.

When you roll a 20 on an attack roll made with this weapon, you can choose to cast mental prison on the target you hit if the target is a creature (save DC 15). Once this effect has been used, the weapon can't be used this way again until the next dawn.

What is the rarity of this magic item? I am expecting this will have a high rarity, but I'm hoping it's not as rare as legendary, so this means I want the weapon to be very rare at most. My main concerns are the +2d4 psychic damage and the casting of a high level spell on crit. Do these effects push it into legendary? If that's the case, what could I do to push it back into very rare?

• Suggested names for your bow: The Barmy Bow, Lance of Lunacy, Piercing Paranoia, Neurotic Needler. Alliteration works well for magic items I have found. – Slagmoth Nov 26 '18 at 13:59

Compared to two very rare items, this one appears to fit very rare

The sixth level spell mental prison is quite powerful. Let's walk through the power comparison with two other items that has multiple features after we look at your proposed magic item.

Brain's Bane (suggested name)

1. Magic Weapon (no +1 to hit or damage) makes that element about the same as Common.

3. 2d4 psychic damage is like some rare weapons (dagger of poison, 2d10 poison damage with a save, but it adds the poisoned condition). Flame tongue does 2d6 fire damage. So we are up to rare.

4. A level 6 spell once per day.

This very similar to the Warlock level 11 feature, Mystic Arcanum, but not as powerful since you need to score a critical hit for this to work1.

Comparison 1: Staff of Power.

This item is very rare and requires attunement. It offers fireball at 5th level (4x per day), or cone of cold (4x per day) or 5th level lightning bolt (4x per day), or hold monster 4x per day. And it's a magic weapon. That's 20 spell levels per day, but only very rare.

Brain's Bane, while quite powerful, does not overpower a Staff of Power. I'll argue that it is comparable to a multi-featured very rare item.

Comparison 2: Cloak of Arachnida

(Wondrous item, very rare (requires attunement)(Ref SRD, p. 214)

1. resistance to poison damage.

2. climbing speed equal to your walking speed

3. move up, down, and across vertical surfaces and upside down along ceilings, while leaving your hands free.

4. can’t be caught in webs of any sort and can move through webs as if they were difficult terrain.

5. use an action to cast the web spell (save DC 13). The web created by the spell fills twice its normal area. Once used, this property of the cloak can’t be used again until the next dawn.

Web is a second level spell (once a day). The various other powers are similar to either a class feature (resists poison) or a variation on the Freedom of Movement spell (4th level, but restricted to webs) and a spider climb (level 2 spell) like ability ... available all of the time.

Brain Bane's once per day 6th level spell doesn't take you into legendary. The 2d4 psychic is less than flame tongue's 2d6 fire. Very rare fits within that relative power level if you compare it to other very rare items.

Suggested modifications

@BenBarden's comment is worth summarizing here: the 6th level spell only activating if you roll a 20 is not as flexible as being able to cast that spell once per day. The critical hit requirement makes it both highly unlikely that it'll show up when it would be most useful, and possible that you won't be able to cast it at all during a given day. You'd be fine making the mental prison spell trigger on any hit, once per day, rather than only on a critical hit. That would not overpower the item, per the above comparison. Alternatively, having it trigger on any critical is still not that likely, and below the Staff of Power's high end very rare power level, depending on how many time your character attacks and hits with that bow in a given day. It would take some play testing, but if you allowed it to cast on any critical hit, up to three times per day, that's still below the 20 spell levels of Staff of Power's various at will 5th level spell castings.

If you wanted to slide this down to rare, the 6th level spell would need to be downgraded to a level 1 or 2 spell, which would get you in / around the power level of flame tongue.

1Coda: @Slagmoth made some nice suggestions for naming this bow in a comment, that I thought were worth including to round out the answer.
The Barmy Bow, Piercing Paranoia, Neurotic Needler, or Mindbane per @Stackstuck's suggestion.

• My suggestion would be Mindbane rather than Brain's Bane. – Stackstuck Nov 27 '18 at 7:39
• I keep misreading the bow's name as Brian's Bane. – T. Sar Jan 31 '19 at 12:18

I had a brief comment, but perhaps an important one. The mechanic seems unusually effective from an action-economy sense. The other answer compares this to staves of power, etc. that require an action to use.

A Rogue Assassin can (essentially) guarantee a critical hit on the first round, which means they start every battle with +4d4 psychic and a level 6 (5d10 psychic minimum) spell at no cost to them.

There is no decision needed to make this attack vs using an action, it just comes along as ~50 psychic damage for free.

I still believe "Very Rare" is fine for this weapon, as the lack of +X on to-hit/damage is a good counterbalance, and the psychic damage is mostly flavor when not a critical.

• Slight counterargument, the weapon description says "When you roll a 20 on an attack roll made with this weapon", so an Assassin's autocrit won't trigger this unless they roll a 20 on the die. That detail might invalidate this answer somewhat. (Also, no such thing as "thread nerco" on stack exchange, so don't worry about that.) – NathanS Jun 12 at 21:08
• Do you think that Very Rare is a valid rarity, or a different one? – KorvinStarmast Jun 12 at 21:23
• Ah, I'm sorry, I read the "and on crit" from the description without looking at the formal implementation. – Cireo Jun 12 at 22:10