From the introduction:
Ostensibly brothers, these eight troublemakers appeared one day in Rivermarch City and immediately began causing trouble for the highest bidder. No one is sure what they are: rumors say they are gnomes, demons, spirits, sprites, and many other preposterous things. Alone, an Octuplet is weak and almost powerless, but the more Octuplets there are in an area, the more powerful each individual is. Each Octuplet has its own set of abilities, but when two Octuplets occupy the same area, they share each other’s abilities.