The River Lethe

The River Lethe is a powerful spell, capable of altering another being's magical capabilities.

The basic use of this spell is to make your target lose the ability to cast any one spell of 1st-8th level. You don't need to meet any requirements in that spell, including costly components. You do not need to know the name of the spell, as long as you can describe its effects to the DM based on what your target's done with it before. Your target will cast the spell, expending a slot, but with no effect.

If you try to make your target forget a spell they do not have prepared or do not know, the spell will not work and you will not lose a spell slot.

Alternatively, you can create one of the following effects of your choice:

• Your target will lose the ability to cast a cantrip of your choice for an hour.
• Your target gains disadvantage on any one type of check/roll/tool/weapon/etc. for one hour, effectively forgetting how to use their abilities. Your target also loses any applicable proficiency.
• Somewhat like with the Wish spell, you undo a single recent event impacted by your target by forcing a reroll of any roll made within the last round (including your last turn) by or at your target. Reality reshapes itself to accommodate the new result. You can force the reroll to be made with advantage or disadvantage, and you can choose whether to use the reroll or the original roll.

The stress of casting this spell weakens you. After enduring that stress, each time you cast a spell until you finish a long rest, you take 1d4 necrotic damage per level of that spell. In addition, your Strength drops to 10, if it isn't 10 or lower already, for 1d6 days. For each of those days that you spend resting and doing nothing more than light activity, your remaining recovery time decreases by 2 days. Finally, there is a 25 percent chance that you are unable to cast River Lethe ever again if you suffer this stress.

To cast, wet a finger in water or an alcoholic drink and point it at your target. The target must make a Wisdom or Intelligence saving throw at the DM's choice, at disadvantage. On a successful saving throw or the spell's interruption through Counterspell, the target will suffer no ill effect and the caster will suffer the above noted ill effects.