Hell's Bells.

One thing that is still unknown (by everybody except Kyng) and that Fabian surely is not happy about and doesn't speak of lightly, is that he has a very serious heart problem coming from the fact it is heavily damaged and since it happened many years ago, by now there is no way to fix it, assuming there could be a chance to do so even back then.

When he was on the street, actually the same exact evening of the day in which he went to beg his brothers to donate that bit of money to him to craft his prototype and have the chance to be forgiven by their father, his rotten luck reached the top.
While he was all curled up on a half dry spot near the street -since it was Autumn II and the city was literally into a lake of mud-. trying to fall asleep and to forget that woeful day, an old and rather mad, uber drunk tramp arrived. There was a couple more of those there, who didn't move nor rose a finger, just watched.
The drunk man started to hit and accuse Fabian to have stolen his spot, and the young Riddle had absolutely no intention to let him mistreat him and steal that spot from him out of a ridicuous statement. He argued back and it became quite fiery, he couldn't imagine that the other tramp had a knife and even less that he would use it. In that situation, he got stabbed and right through the heart. As soon as that happened, all of the tramps including the drunk one fled, leaving him alone to die in the mud, and he was absolutely sure it was over for him, he fell on the ground and tried to keep his eyes open, but he couldn't manage to, he passed out in minutes.

It is totally mysterious how he managed to survive but probably some Zimox had something to do with it, it was probably not his time to die, he had still to fullfill his purpose back then.

He woke up hours later, alive, to his greatest surprise and with the wound on his chest stitched up in a very poor, dirty, cheap and messy way, but at least he wasn't bleeding anymore. Though, the damage inside, that of the heart was there and already paining him badly.
He took it as the definitive proof he had to leave the North forever, and also what makes him feel the worst towards his brothers nowadays: had even just one of them opened his heart to him that day, his heart would have never been damaged and he could live a full life. He can't help but blame them for it.

His heart problem causes:
-Very unstable heart bits;
-Very weak heart, great and constant risk to get an infarct;
-Must take medicines his whole life to reduce the risks; (and the medicines he takes would insta kill him if he drank any sort of alcoholic drink after or before taking them. For that, when he knows he will drink, he avoids to take them, making himself very vulnerable.)
-Must avoid to run, do sports and such as it would bring too much effort to his heart; (even stairs are an actual problem for him)
-Avoid excessive stress. Spook him is definitely a bad idea.

The last evening he spent to the North for the fair before having to leave for a conference in the South the next morning, something messy happened.
He stayed till way too late in a pub, drinking with his fans. (In the 3 days he didn't take his medicines once)
He went out of it alone, totally unaware of the fact that Benjamin and William were waiting for him there.
They didn't have any ill intentions obviously, they only wanted him to be forced to listen to them to fix things, since he never let them approach otherwise. And neither they could imagine of his heart problems, heck they all are convinced that Fabian faked his death, it was the most reasonable thing to believe after he appeared on all newspapers a fake name. (Since investigations were made and the tramps who witnessed the stab had their minds read. Their conclusion though was that he had some way to change their memories considering he was alive all the time and that blow would kill any Auriold. Needless to say that situation just contributed to create even more tension as thinking he faked his death pissed everybody off big time. But they are going to find out the truth very soon.)

As he went out of the pub, Ben and Will approached him, but very childishly, he immediately started to run away, quite spooked by their attempt.
No medicines plus drunk plus scared and agitated plus run are the most lethal mix for someone in his state.
The brothers just facepalmed and gave up, going away while he headed to some dark alley in which he collapsed, in the middle of a heart attack pretty deadly. Luckily for him a tramp nearby noticed the scwne and started to call for help, dragging the attention of someons who knew what to do. Basically, doctor Robstein was called with urgency and Fabian was transferred to his clinic where the doctor saved his life and then moved him to his own mansion for monitored convalescence since he is still at risk.
And it is while there that Christoph gets to learn about the truth about Fabian.

Christoph was summpned by the Master Doctor to be warned they will have to postpone the upcoming exam of a member of the Academy of Medicine due to an urgent case he can not leave unmonitored.
They spoke right in front of the door of the room in which Fabian was. And it was a luck as while talking, the two heard a wprrying noise from the room and stormed in, finding Fabian on the ground undergoing another cardiac arrest.
Loris and Christoph performed the delocate surgery together, in fact the Anatomy professor was the top cardiologist pf Nortrig and despite he is no longer a doctor he still is the best. (Loris Robstein is the top expert is transplantations while Jackil the top pediatrician)
They managed to save his life a second time, but the whole thing left Christoph upset like never before.