This table was made to give at least a little bit of flavour to the effects, or flesh them out if you're a bit fast and loose with balance (which you should be).
Bestow a curse...
Flip a coin to decide if the effect is real or only imagined. (Effects in brackets could potentially replace the d8 necrotic damage on a hit on the cursed target. These effects would normally last until the end of the target’s next turn if the caster hits them with a weapon or a spell. These are permanent effects for more powerful curses, such as those cast using 8th or 9th level spells slots.)
A Bestow Curse cast at 5th level or above can apply the following alteration to the Curse then apply the following to the target:
The curse cannot be dispelled or removed by any means, except by one that is declared by the caster. This must be literally and physically possible for the target to accomplish within the duration of the spell, or the spell fails. Once the conditions are met, the spell ends immediately.
This curse means that:
While affected the target has disadvantage on:
The alternate effect is: