Victory on Janus — Andre Norton

DEATH STRUGGLE
The Ifts of Janus had once been human. Transformed, each shared his human memory with that of an Ift. But the Iftin memories were stronger, and left the changelings unprepared for the danger which faced them in the form of the unnameable THAT WHICH ABIDES.

That dreaded evil was amassing a human army to take over the planet. To Ayyar and the other Ifts, the enemy was too powerful, too insidious, too utterly alien to face.

As the chosen warrior, Ayyar had to seek out the heart of the enemy and destroy it, before it vanquished the Ifts for once and for all.