A Clash of Kings
A Storm of Swords
Gregor is dying, with his flesh rotting and the wounds oozing pus. He screams constantly, has violent convulsions, and the veins in his arm are turning black. Grand Maester Pycelle is unable to determine the substance that could cause this, and cannot heal him despite his efforts. When he leeches Gregor, all the leeches die. Pycelle tells Tywin Lannister that he does not believe the poison could be manticore venom, because of that poison's usual instantaneous effect.
A Feast for Crows
Queen Regent Cersei Lannister has the former maester Qyburn examine Gregor. Qyburn reports that Gregor is slowly dying in exquisite agony; all of Gregor's veins have turned black, and the poison has eaten a hole in his side as large as a fist. He believes the poison was in fact manticore venom, but thickened somehow, most likely with a spell, to prolong the Mountain's death. Qyburn convinces Cersei to allow him to move Gregor to the black cells, where his screaming cannot be heard by the rest of the Red Keep, and study him there to gain useful knowledge of the sorcerous poison.