The Underhive is Cain's element. To say he's 'happy' down there at the moment would be untrue, because his life is still at risk, but it's an environment he knows exceptionally well and feels 'comfortable' in. It's about as ideal a battlefield for him as he's ever likely to find. He's going to be blending into shadows, ambushing his enemies, and leading pursuers on a merry chase through the labyrinthine depths. He'll be impressing his new vampiric allies in the process, and his reputation will grow still further.
I now have the image of cain and mannfred confronting each other and cain doing bane's speech like in the drak knight rises
Cain: "Oh, you think darkness of the underhive is your ally. But you merely adopted the dark of the underhive; I was born in it, moulded by it. I didn't see the light of the sun until I was already a man. And by then, it was nothing to me but blinding"
the duel continues and mannfred tries to attack cain from the shadows but fails
Mannfred : 'HOW CAN YOU KNOW WHERE I AM?'
Cain : "The shadows betray you because they belong to me."
Mannfred tries to use ilustion and mental powers against cain, but cain gives the impression that it doesn't work on him.
Cain : "Theatricality and deception - powerful agents to the uninitiated. But we are initiated, aren't we Mannfred?"
Mannfred is frightened and starts to back away
Mannfred : " I'M NOT AFRAID OF YOU!"
Cain: "Good, because now is not the time for fear... That comes later."
In the meantime, Malicia is having rather complicated emotions because of the show that Cain is putting on and the vampires are starting to feel nervous and are wondering if Cain might be some kind of super vampire.