Free drama. xD

What Sao showed Benjamin in that evening at the club? The whole scene in Edward's mansion, from when they arrived to when they left.

Christoph ran all the way there, following his son and brother, in panic.
As soon as they arrived, Edward led Bart to the basement where he chained him. Christoph followed down and kept begging Edward to not do it, without purpose. So, Christoph asked to have a few words in private first and that was granted to him.
He tried one more time to beg Edward to spare his son from it, that he would make his son understand and regret his mistakes, but to that, the brother simply turned around to head back down. He stopped only at Christoph's next words, begging him to take him instead of Bart, saying he claimed his punishment on himself.
At first, it was clear that Edward was not pleased. He shut his eyes for a couple of seconds and tried to discourage the younger brother from doing it. But it was pointless, it was clear that Christoph could not bear the idea of his son being tortured. It would hurt and affect him more to know his son being tortured than to be tortured himself, which is the main reason why Edward was forced to accept. It was Bart the one meant to suffer not Christoph.
Edward finally slightly nodded and walked downstairs again, followed by Christoph. There, he went to prepare his tools while the father stood in front of his chained son, starting to undress himself (just top of course). To that, Bart started to guess something and to panic for real. He had accepted his punishment, he knew he deserved it and it would be a sort of relief to him. But this was not what he wanted.
Bart started to shout to Edward to let his father go,  that this was unacceptable, he started to struggle to try to free himself but everything was vane. The magic chains nullified his powers and drained his strength, he had no way to free himself. To that, his father even tried to tell Edward to not force Bart to witness, but of course it was ridiculous.
Once finished to undress himself, Christoph spread his arms, thinking Edward would chain him to, but he did not, of course. Christoph was here by his free will, if he wanted it to stop, if he wanted to leave he had the right to.
Before starting, Edward gave some last words to the brother "Beg me once and I will stop, with you." clearly letting guess he'd move to Bart then.
Christoph nodded and shut his eyes for some moments. His fear was clearly evident, he was trembling uncontrollably but was determined to keep his son unharmed.
Edward started with his usual injection, of a specific substance of his creation having the effect to sensitise the body extremely, acting on the nerves so that everything hurts hundreds of times more than normal. With that in circle, even a slight touch is felt as painful. And he injected the black liquid straight through Christoph's heart, which alone was rather grim to feel, even though totally painless. But after that, the hell started.
For a few minutes he tried to restrain himself from screaming and showing any sign of suffering. Then in the following hours he screamed to the point that even that was too painful. When he had no longer the breath to scream, he started to cry. He felt humiliated beyond measure by this. But he never begged to stop.
At some point he was so broken he could no longer stand by himself anymore and started to feel that he would faint. He warned Edward that his pressure was dropping, he even gave him some confused advices on what to do to prevent him from fainting, but Edward did nothing. When Christoph felt and did not manage to stand up, he simply helped him and supported him, while continuing to torture him. Meanwhile, Sao forced Bart to watch and told him cruel words on how that was all his fault, he caused his innocent father to suffer so much etc.

After 5 endless hours which felt like a whole eternity, Edward glared at Sao so that he would come to support Christoph and left the brother. It seemed to be over, but there was the final part, the care of the wounds, which with Edward doing, was just another cruel torture.
After thinking it was over, when Edward approached him with a vial containing a very familiar disinfectant, the strongest one, used only on anesthetized patients, as it literally burns like acid on the wounds. At the sight of it, Christoph almost let out a desperate "nooh" but restrained himself, fearing it would lead to Bart living through the same.
He kept crying till the end and only a couple of minutes after everything was over for good, Christoph finally passed out. Which gave Sao some serious and worrying doubts that his terribly bad luck might not be natural. After all, it passed about 1 hour and a half from when Christoph reported the risk to faint soon, before he actually fainted, which did not grant him to be spared from even just the mending of the wounds. And no, Edward was not behind it, Sao did not even need to ask. It was clear he wanted to let him faint, as he did not take any action to prevent it from happening, which increased Sao's suspects even more.
Anyhow, in the end, Bart was freed and told to dress his father up while Sao kept supporting the fainted man. Afterwards, they both were led back to Christoph' s home where Bart took care of his father.

As in Edward style, Christoph's wound were taken care of so that no scar will be left with the exception of one, he leaves on purpose as a true signature. A scar of the shape of two short lines one above the other, like a = a few inches under his chest, on the right side of his body. Which is pretty noticeable (when not wearing top clothes, that is)   
It took Christoph just a day to recover completely, physically. He may never fully recover the trauma, but that will only involve him having some scared reaction to: Edward; syringes; Yiyvt disinfectant. And given the subjects he teaches and his duties of assistance to doctors, they can lead to some slight troubles, but nothing major.