Dark Omen

You touch a creature, and that creature must succeed on a Wisdom saving throw or become cursed for the duration of the spell. When you cast this spell, choose the nature of the curse from the following options:

• Choose one ability score. While cursed, the target has disadvantage on ability checks and saving throws made with that ability score and it decreases by 2, to a minimum of 1.
• While cursed, the target has disadvantage on attack rolls and its AC decreases by 2.
• While cursed, the target must make a Wisdom saving throw at the start of each of its turns. If it fails, it wastes its action that turn doing nothing. If it fails 3 times, the target's Intelligence and Charisma become 3.
• While the target is cursed, attacks and spells deal an extra 2d8 necrotic damage to the target.

If you mantain concentration for the full duration, the curse becomes permanent. A creature can' be affected by more than two curses of this nature at the same time.
A greater restoration spell at 8th level or higher ends this effect. At the DM's option, you may choose an alternative curse effect, but it should be no more powerful than those described above. The DM has final say on such a curse's effect.