After digging a hole in the dead center of a set of crossroads and burying a box containing a picture of the mortal wishing to make the deal, some graveyard dirt, and a bone from a black cat a demon can be summoned [source].
This demon takes a form most acceptable by the summoner.
Then the deal is made. For the summoner's soul the demon will fulfill one wish. The soul will be harvested in the end of 10 years.
Or that is what the demon says. Demons are liars. They want souls. They don't value pacts and agreements they do with lesser beings. They say they'll get your soul after 10 years, but hours, days, years, who cares. They will collect your soul in 10
When it is time to pay the dept demon sends a creature called The Collector to kill and devour the soul. The soul is now demon's snack to feed. No resurrection of any kind possible. Ever.
Hit Dice: Character's level +5
Armor Class: 4 
Attacks: 1 (Gore 2d8 or special)
Saving Throw: F18
Special: If Gore attack deals 10+ damage it's saliva affects the AC of its victim reducing it with 2 points.
Alignment: Lawful Evil
The collector is invisible. Only it's hellish drooling might reveal it's position. One easier to hit than normally invisible monsters. It's vision is reversed. It sees in pitch dark and gloomy as in broad daylight, but broad daylight it cannot see at all.
If it's Health Points are reduced to 1/4 of original it will flee. Next time, in 1d4 weeks, it is recovered and returns with another Collector. This happens as long as the soul is collected.
The Collector prefers to hunt those who are currently alone. If you want to save your butt, or more likely prolong the inevitable never stay alone. Always be in a company. But eventually, it will catch you.
|Dirt Cross Roads Alex j coyne|
Inspired by: Robert Johnson, Supernatural