From Ardrana

Magical nexuses are places on Ardrana where magical energy is concentrated. It is not known how many of them there are, but many have been located and charted by sages. The effect of these spots is that the closer one gets to one, the more powerful the effects of one's magic. At the nexus itself, such power is doubled or even tripled if it is powerful enough.

Sages have speculated that the nexuses (or nexi, as some insist on calling them) are the remnants of what was once a more powerful magical field that encircled Ardrana. Some of these theories tie the waning of that field to the destruction of the Purple Castle, or to the magical calamity that caused a large section of eastern Rannas i'Drasoin to be submerged. These are, for the moment, only theories.