Star Born (1957) by Andre Norton

Mark F. Smith wrote:Andre Norton's "Star Born" pictures a human colony in another galaxy, driven away from Earth generations ago by a repressive government. Considered outlaws, the colonists are in permanent hiding.

They have developed friendship and cooperation with a local race of "mermen" who are equally at home on land or sea. But that race only took to the sea to escape a malevolent power that hunted them and killed them violently for sport - Those Others.

With a global decline in the population and reach of Those Others, contacts are few and the humans have no direct knowlege of them. So it is a major surprise when Dalgard, a human scout on his coming-of-age expedition, along with his "knife-brother" Sssuri of the mermen, run into a party of Those Others who are bent on reclaiming hideous weaponries left behind in one of their abandoned cities... and find that they are being aided by new arrivals from Earth!