Nexus Points

There are places in the world that are different, to say the least. Regions that defy the natural laws and are both wonderous & devilish in the one go. These are usually referred to in Occult circles as Nexus Points, regions where there is wild untamed magic that spill out affecting the very fabric or reality. Many are tied to portals between different dimensions, and others where magic has infused with the earth, or water in some cases, even the very air. Sometimes created in ages past by the use of powerful magic, in ritual form, or in ferocious battles between opposing mages, each can be tapped by creatures that have the right powers.
Many of these regions are thought to connect to Ley Lines that overlap. These mystical lines supposedly thrum with magical power and each time when they connect, the power increases exponentially. Many creatures feel the power in the region, even if it is a slight tingling or miniscule electrical charge. Most are avoided as the chaotic energies that can build up can shape reality in such a place to the detriment of living beings, and even undead & constructs. Sometimes, if a portal is present, creatures from other dimensions can force a part or the whole of themselves through to the Material Plane. A number can be crossed from the Material Plane to worlds nobody could truly believe, and in a few cases, even worlds in the Material Plane itself.
Ancient myths held by some of the eldest races talk of them being turned into doorways between different spheres. Some refer to technological breakthroughs stabilising a few and turning them into devices called Stargates, while others are rumoured to have had parts plundered. In the latter case the rumours go on to say elements of the saturated material found in & around a Nexus Point have been plundered, stabilised, and built into small devices requiring sequences or puzzles to be solved. These open doorways between worlds on a temporary basis, and can be carried in the palm of one's hand.
Exactly what or how these formed can never be truly known, at least in the majority of cases. Most are thought to have formed naturally at the creation of the planet, and a few created through deliberate means, and a few accidental over the millennia. Very few are known about these days, many being lost to time or flickering out when the energy of them fade, plundered, or destroyed by cataclysmic events. A few times the remains are found and activated again, either through the use of magic or some prophecy that foretold of the event happening. Sometimes there is a residue that is left behind in the ruins of such a place, usually in the form of extremely rare and invaluable substances. These range from warpstone, an amber-like substance (like the material in amber sarcophagi), a black substance similar to solidified oil (a tad greasy but does not burn), to two types of metal (adamantine & mithril). 
Nexus Points that still exist seem to be found primarily in out of the way places in any environment, including in mountain passes, ocean depths, deserts, forests, grasslands, and even deep in the Underdark. A few are rumoured to be deep below, or built into, buildings in a few towns & cities. If this is true, the correct events that can open one again could potentially spell disaster for the settlement's inhabitants. A few of the settlements could have been built deliberately to serve as protection against such an occurrence of one opening, or even the inhabitants looking for a way when it will open and bring about more power to themselves.
A few of the eldest living Dragons, Giants, Star Gods & Spawn, & Yuan-Ti, along with others like elder mages, druids, and clerics across the world has any real chance of knowing of at least one of these Nexus Points. Many do not share this knowledge, at least willingly, and quite a few would die before telling. This is mainly that if sufficient Nexus Points are known, and tapped, it could plunge the world into anarchy with the beings of other dimensions, and also the sudden increase of magic poring out, would lead to cataclysmic events, saturating the world in magic, and warping the world into absolute madness. If nearly all are fully found and activated, the world could face annihilation on a scale that would destroy it, and the rest of the solar system.