Blackball

This five foot diameter ball of black energy slowly moves toward you. Everything in it's path that it touches instantly disintegrates. 

Source: AD&D 2nd Edition Mystara Moustrous Compendium Appendix

     The Blackball is also known as the Deadly Sphere. A ball of black energy that destroys everything in it's path. This rare creature is immune to almost all forms of attacks. When a Blackball appears it is more of an event then an encounter. How do you contain this destructive force? Simply rushing into combat will result in death.
     There are a few theories of the Blackball existence. Some believe that a powerful fiend was captured by Immortals and transformed into this deadly orb. Although some scholars have debunked this theory since Blackballs are neutral in nature. A second theory is an unknown race created this creature to transport victims to another plane of existence. Scholars also have doubts since it appears the individual is quite obviously disintegrated.
     The most popular theory is that the Black Balls were created by Old Ones. Outer Gods of immense power that easily surpasses any mortal deity. Thankfully the existence of a Blackball is very rare.

Combat: The Blackball is worth 8,000 experience in AD&D 2nd Edition. It has no HD and an AC of 10. It is one of the few creatures that doesn't actually have hit points since everything from melee attacks to spells have no effect on it. It doesn't avoid attacks and wants to be hit since anything touching it simply vanishes.

-Disintegrate: Any magical or non-magical object or person who simply touches the Blackball is destroyed. There is no saving throw. You are dead. Your items are gone. Life sucks. Only artifacts at the DMs discretion might survive being touched by the Blackball. Since the Blackball is extremely slow(only 3 movement) any player character can attempt a Dexterity check to avoid touching this deadly sphere. Best tactic is simply evading this creature or running far far away.

-Lifesense: The Blackball senses all intelligent lifeforms within a 60ft radius. It randomly chooses a victim and moves in a straight line to destroy that individual. Those who move beyond the 60ft radius are not detected. The Blackball can simply detect you from an upper level or lower level as long as you are within 60ft. It can drop down from the ceiling destroying all the stonework that is in it's way.

-Spell Immunity: Blackballs are immune to all spells with the exception of a couple. A very carefully worded wish might suprress the Blackball. Also a gate spell cast within 60ft of the Blackball will have it gravitate towards the portal and head off in a new domain. Now you might have trouble if you piss off another world since you sent a ball of destruction their way.

     Blackballs aren't really meant to be a random encounter. A whole campaign can based around the appearance of this orb. This event can spawn a number of side adventures to find a way to contain or destroy this threat. Conventional means are simply ineffective and the PCs must be resourceful in figuring out a solution. Maybe seeking an audience of the Gods for advice or finding a long lost artifact to halt it's destructive path.