"What happened to him was a tragedy, as I told you before that to advance in this technique you need to not only have your flame but also the toxicity of the core to advance.
Advancement of the flame is still considered a possibility as there are various methods since many other cultivation techniques also require an advanced quality flame for their advancement.
As for the toxicity, it is one of the subject that is hard to advance in as to increase the toxicity element in the cultivation requires the consumption of things that contain high toxicity in them.
Now, if what you ingest is not been able to be digested by your cultivation then the adverse effects that it would bring are not something to laugh at.
The adverse effects it would bring are that first, it would start to consume your foundation as a form of balance but instead of it forming a balance it would slowly start devouring your foundation to increase itself.
What you might not know is that any form of poison also contains a life of its own when it is of a being or a creature equivalent to the Form embodiment realm.
Even after the creatures death, there would be a life left in the venom of the creature, it is same for the other elements as well such as fire, ice and thunder.
When the elder disciple had reached the peak of the Spirit refining realm and wanted to breakthrough to the Form embodiment realm he had to first advance the Lethal Flame Core, for that he had bought the spirit venom of the Abyss viper to advance his core.
As the Lethal Flame Core technique has strict requirements it also has quite a bit of good points, one of which you have just discovered while the other is that any kind of venom or toxicity that is injected into the core would become a part of it and it would also contain the venom's properties.
For this reason, the elder disciple had chosen the spirit venom, in the future when he attacked his opponent, hsi attack would not only contain the power of high-grade flames but they would also attack the opponent's soul.
From what I remember the elder disciple had his whole family killed by our enemies, for this reason, the elder disciple was always trying to get his revenge for this reason he was always eager and for this reason he took this step.
As you can understand what happened afterwards, he was not able to digest the venom and was slowly digested by the venom itself. He was slowly replaced by the Abyss viper spirit that was present inside the venom and assaulted the other disciples in the academy, the elders in the Academy couldn't sit silently at this and was killed" Elder Fryer said with a sigh
This had been one of the tragic history of the Flood dragon Academy, many of the current elders we're part of the batch that was under the regiment that took place after that incident happened.
After that incident, the higher-ups of the Academy paid close attention to this kind of people that would come close to the third realm and needed mutated elements for their breakthrough.
Even elder Fryer had to go through their checks before he was allowed to consume a mutated fire element to break through to the Form embodiment realm.
After listening to the story Knave went into a deep thought process as he had not thought that there would be this kind of problems and requirements to advance to the third realm.
He could somehow advance the toxic level of his core with his speciality as for the equilibrium required with the flame, he could start his alchemy practise and that would help him in learning how to maintain the balance.
He still felt a little relieved because he could still break through to the Spirit Refining realm without much trouble.