I was inspired by the Thoughtseize card from Magic, the Gathering to create a spell that achieves a similar effect.
I intend to introduce this spell as something that must be studied extensively at an academy, is considered "new magic" but also probably taboo, and likely as something wielded freely by a villain or cabal. The PCs will be able to learn this spell eventually, but they can't just choose this spell when leveling their character unless they've done the necessary preparation. I haven't worked out what that is yet.
The spell is intended to be available to Bards, Wizards, and maybe Warlocks.
The spell description:
Casting Time: 1 action
Range: 30 feet
Components: V, S, M (a needle or pin)
Duration: Concentration, up to 1 hour
You probe the mind of your target, and can sever a thread of their memory for a time.
Choose a creature you can see within range. The creature must succeed on a Wisdom saving throw. On a failed save, if the target can cast spells, the affected creature forgets one spell that is innate, prepared, or otherwise fixed in its mind that is one level lower than the spell slot used to cast this spell. You may choose which spell is forgotten from a list of valid choices. If no spells meet these requirements, you may choose a spell of a level lower than the level used to cast this spell, but of the highest level the target can cast. Otherwise, this spell fails.
When you cast this spell using a spell slot of 4th level or higher, you may cause the target to forget an additional spell for every two levels the expended slot was above 2nd. Additionally, for each spell you can choose beyond the first, you may select a spell up to a number of spell levels lower than the level used to cast this spell, equal to that spell slot level minus 1. You must announce the spell level(s) from which you wish to select before selecting any spell(s). You may only select a single spell from each spell level.
This effect lasts for the duration of the spell. While concentrating on this spell, you suffer 1d4 psychic damage at the beginning of each turn due to mental strain. This damage cannot be prevented by immunities or magical effects. If you use this spell to cause the target to forget more than one spell, you suffer an additional d4 of psychic damage for each spell chosen beyond the first.
I am looking for any and all feedback, but I am primarily concerned with whether this spell is balanced for PC use and/or for Villain use.
One phrase I'm particularly unsure about is from the first paragraph:
If no spells meet these requirements, you may choose a spell of a level lower than the level used to cast this spell, but of the highest level the target can cast. Otherwise, this spell fails.