Black Hole

Summary
To quote from Wikipedia:

"A black hole is a region of space in which the gravitational field is so powerful that nothing, not even light, can escape its pull after having fallen past its event horizon. The term "Black Hole" comes from the fact that, at a certain point, even electromagnetic radiation (e.g. visible light) is unable to break away from the attraction of these massive objects. This renders the hole's interior invisible or, rather, black like the appearance of space itself."

Black holes in fiction are some of the trickiest things to analyze, because it is often difficult to determine if it is a "real" black hole, or just some kind of black hole-ish void. Since black holes in fiction rarely conform to the actual physical properties of their real-life counterparts, this is usually a problem.

An actual black hole, in a fictional context, is generally considered to inescapable to any entity that does not have either reality warping, FTL speed, or time travel, but the actual properties of one may also have been ignored due to plot conveniences or bad writing.

Small black holes produce gravitational shearing forces capable of tearing apart any kind of matter at the subatomic level (of course, within fiction, there are exceptions). Black holes cannot be destroyed with brute force, because they will simply absorb all energy/matter thrown at them, and grow larger. They are capable of easily destroying planets and stars (although with small black holes, it might take a while).

Black holes do decay over time due to Hawking Radiation, so if you leave a black hole alone and don't feed it anything, it will eventually grow smaller and explode (this type of detonation is a 100% efficient mass-to-energy conversion, with more potential power than a nuke or antimatter explosive). Tiny black holes with masses less than a few trillion tons would evaporate nearly instantly.

Black holes with higher masses last much longer: A black hole with the mass of the sun, for example, would last for billions or trillions of years before evaporating.