The Intruder

You touch a creature and inject an illusionary parasite into their psyche. That creature must succeed in an opposed Int check. The caster adds their proficiency modifier to the check.  The caster adds +2 for each spell slot above third level used.  The duration is concentration, up to a minute. When infected, the creature is immediately poisoned and must make an Intelligence saving throw at the beginning of each turn or take 1d4 psychic damage on the first turn, increasing by another 1d4 for each failed save.  

Additionally:

• On the first failed save the creature's speed is reduced by 10 feet.
• On the second failed save the creature cannot take reactions.
• On the third failed save the creature is blinded.
• On the fourth failed save the creature is paralyzed.
• On the fifth failed save the creature falls unconscious but is stable.

A remove curse or greater restoration spell ends this spell. A lessor restoration spell removes one of the effects. If the afflicted creature ends their turn more than 120ft from the caster, the spell ends.

At the DM's option, you may choose an alternative effect, but it should be no more powerful than those described above. The DM has final say on such a curse's effect.

At Higher Levels. If you cast this spell using a spell slot of 4th level or higher, the duration is concentration, up to 5 minutes and the damage increases to 1d6. If you use a spell slot of 5th level or higher, the duration is 1 hour and the damage increases to 1d8. If you use a spell slot of 7th level or higher, the duration is 8 hours and the damage increases to 1d10 and does not require concentration to maintain.