World of 'police' killed, one saved and pursues perpetrators [duplicate]

Question

I read it as a young teen (so early 80s). It was (I think) a four book series. It starts with an attack on the home-world of the 'Interplanetary Police' - there was a name for them, but if I could remember it I'd probably have the title :) - with a radiation/poison that ate into the bones. All were killed.

The protagonist was rescued by a kindly old wizard (sorry, scientist), had his bones replaced with an unbreakable material (to remove the poison) and given an ally / side-kick of a flying batlike (BIG bat) alien who may also have been telepathic (bit fuzzy on that) and sent after the baddies.

I think there were 4 in the series.

Answer 1

This is almost certainly Douglas Hill's Last Legionary series, which I also gave as an answer here: Young-adult science fiction novel series. Kid trained as warrior, all other kid warriors wiped out

From Wikipedia:

As the book begins, Keill Randor is the leader of an off-world Strike Team, far from home. He receives a distress signal from his home planet Moros, stating that they are under attack from unknown forces. Keill's ship has suffered damage, and he is unable to immediately return to Moros. When he arrives, the entire planet is covered with radiation. He is met by Oni's ship, and she tells him to flee the planet; a deadly radiation is killing everyone who comes close to the surface.

...

The Overseers also explain that, since the radiation sickness had settled into his bones, they were forced to replace his entire skeleton – using an unbreakable alloy – effectively rendering Keill's bones unbreakable. They also reveal how they captured Keill so easily – a telepathic avian life form from another galaxy known only as 'Glr'.