When the world changed tremendous arcane energies were released across the land forming ley lines. These invisible magic currents rose from beneath the ground and settled in more or less permanent locations. Some have been detected high above the ground, while others pass right through towns.
Most are unaware of their existence, or pay little mind to the fact because the lines are completely beyond detection for the vast majority of the population. Those in tune with the arcane, however, may notice a ley line when passing through one. It has been said that magic may behave differently within a ley line.
A nexus is a crossing of two or more lines. Such points are extremely rare. It is estimated by mages that study them that there are no more than five at or near ground level. These locations are more easily detected by users of magic when passing through them. It is rumored that practitioners of complex ritual magic seek out these locations, constructing temples. It is also speculated that nexuses could be doorways to other realms.