From slave to leader of the beasts

Kalavan

Journeyed there and back again
#1
I’m talking about a book I’ve read many years ago, so the details are quite foggy in my memory, an overall mediocre read, but with a very original and captivating story-line in it:

- It was part of a serie, I think 3 or maybe 5 books long – but I’ve read the italian edition, perhaps the original edition was just 1 or more books, divided in smaller installments;
- Found it nearly 15 years ago in my library, but the book was already quite worn-off, so I would say I’m talking about a book published at least in the ‘80s/ very early ‘90s;
- It followed several different characters; one of them a young warrior from a celtic-like clan, aiming to be a mercenary in his uncle company;
- The uncle –by a distance the more interesting POV – is captured by the anthropomorphic beasts who are invading the world through some kind of portal, and made their slave, together with several half-human/half-beast hybrid; he soon befriends – maybe with some minor romance – a female hybrid, and then leads these slaves in a revolt;
- these hybrids were considered inferior in the eyes of the beasts’god – this religion play quite an important part in the story; after this revolt they gain the respect of a few of the other beasts and are warily accepted as a clan of their own;
- meanwhile the beasts are defeated by the other characters and repelled through the portals; the uncle decides to follow them, in order to lead the hybrids’ clan and protect them from the proper beasts, who still mostly despise them;

Rings any bell?