Spiders in the Arena

So when i was fighting Bloodhammer and he was doing his special bite attack he gained hp even when i was on the other side of the room so not in red zone and didnt get any dmg. No other idea just that he bit those spiders and regain health.
Is this intended?

I swear he eats spiders! There was a leg sticking from his jaws. I saw it!

I love spiders though... die at one blow :)