Monstrous Monday: Imp of the Perverse

"We have a task before us which must be speedily performed. We know that it will be ruinous to make delay. The most important crisis of our life calls, trumpet-tongued, for immediate energy and action. ... It must, it shall be undertaken to-day, and yet we put it off until to-morrow, and why? There is no answer, except that we feel perverse, using the word with no comprehension of the principle. ... The clock strikes, and is the knell of our welfare. At the same time, it is the chanticleer-note to the ghost that has so long overawed us. It flies—disappears—we are free. The old energy returns. We will labor now. Alas, it is too late!"
- Edgar Allen Poe, The Imp of the Perverse

The Imp of the Perverse is a small invisible imp that can be summoned much as any other demon to cause someone harm.    The imp is small, no larger than a rodent really, and invisible to all but those with the ability to See*.
(*Eldritch sight, detect magic, detect curse, detect invisible, or other such magical sights.)

Imp of the Perverse
Basic-era stats
No. Enc.: 1 (1)
Alignment: Chaos (Chaotic Evil)
Movement:  120'(40')
    Fly: 120' (40')
Armor Class: 9 [10]
Hit Dice: 1+1
Attacks: 1 (bite)
Damage: 1d4
Special: Invisible, Suggestion of Harm, only hit my magic weapons
Save: W1
Morale: 10
Hoard Class: None
XP: 50

The Imp of the Perverse is a small demonic creature that can be summoned and attached to another victim. Once done so the Imp will sit on the victim's shoulder while holding onto their ear. The imp will then whisper in the victim's ear convincing it to harm themself.

The victim must first make a saving throw (vs. Spells, see below). If this save is made the imp does not latch on and is free to roam about to find another victim (those victims will then need to save) until it tries and fails three (3) victims or the magic-user that summoned it returns it.

If the victim fails to save the imp is latched on and becomes completely invisible.  No amount of detection on the part of the victim will reveal it attached. Others might.  The imp can only be removed with a Remove Curse, Exorcism or a properly worded Holy Word or Banishment spell.

Until then the victim makes a saving throw every day to see it they do something to harm themself. A failed save means that something occurs that seems natural that will cause them harm.

On the first failed save they will take 1d6 total damage. On each failed save after that they will take an additional 1d6 hp of damage; so 2d6, 3d6 and so on until the victim is dead or the imp removed.
Once an imp has been removed or it's victim is dead will it become visible once again and can be attacked.

Summon and Bind Imp of the Perverse
Level: Cleric 3**, Wizard 3, Witch /Warlock 3  (**evil/chaotic clerics only)
Duration: 24 hours
Range: Special
By means of this spell, the caster can summon an Imp of the Perverse from the Demon Realms to latch onto a victim.  The victim must be known to the casters and some item of the victim must be present when the summoning and binding are complete. The familiarity of the victim is helpful to the caster to prevent the victim from saving against the attack.

Familiarity	Save Bonus/Penalty
Unknown	+3 bonus to saves
Known by name	+2 bonus
Know each other (can identify each other)	+1 bonus
Know well	-1 penalty
Know intimately	-2 penalty
Same blood or kin	-3 penalty

Material Components: The caster needs a personal item from the victim: clothes, a bit of hair, fingernail, drop of blood, and the like.  A bit of copper wire and loadstone.  The items are tied to the loadstone with the copper wire.