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?