A dramatic scene between Geralt and Regis, with the latter going into a torture cage in order to harvest some of his blood.
Geralt must first kill monsters, to induce a blood frenzy in Regis, before taking a blood sample.
The green glow every now and then is the Tesham Mutna Gear taking vitality from opponents who have died. When an opponent dies, you gain vitality from them, so every time it glows green it is an opponent dying. Which is suitably macabre. I think it works on all opponents. The more pieces of Tesham Mutna Gear you wear, the higher the rate of vitality gain.