[FEH] Even from a timeline where he wasn't their teacher, Byleth still manages to impart a lesson to his students
In the recent Forging Bonds, we got a "Fallen" Byleth, which is a 3Hopes version of M!Byleth that is overtaken by Sothis.
What's interesting is that this story has him meeting the 3H lords from Part 1. Byleth is going on a rampage and the lords are in shock, and somewhat panicking by how Byleth is the cause of the destruction.
> Edelgard: It’s true that it’s the professor who is attacking the town, isn’t it?
> Claude: Unfortunately, yes. If we believe our scouts’ reports to be correct, then yes, it is undoubtedly him.
> Dimitri: I cannot believe it... For what purpose could the professor have to be on such a rampage?
And when they meet Byleth, Sothis makes her murderous intentions very clear.
> Byleth: Listen, wretches. You must leave this place if you wish to live. I will not hold back if you impede me.
> Edelgard: With this murderous intent, it doesn’t seem like he’s joking or attempting to intimidate us.
They realize that they have to try and stop Byleth's rampage, but Byleth's power is too great. They manage to make some damage that earned some praise from Sothis.
> Byleth: Hm. Your allies carved a path so you could attach my blind spot? Not bad. I praise you for your effort.
> Claude: Rrrgh! He really is a monster! Tell me one thing, Professor... Why are you destroying the town?
> Byleth: It need not be said. This is me raging at your foolishness!
> Edelgard: What are you talking about?
> Byleth: Filthy insects crawling across the land... Fools constantly at war in Fódlan. The land swims in hatred. Upon the name of The Beginning, I dispense judgement on all who have lost their way!
> Edelgard: What are you saying, Professor?
> Dimitri: This is not Fódlan. You have no reason to hurt the people of Askr!
> Byleth: Corruption fills this land. I cannot allow it!
The 3H lords realize that this isn't actually Byleth talking, cause Byleth is not someone who would make such grandiose remarks. Byleth prepares to attack again, and the three know that despite how slim their chances are, they gotta stop him.
However, they are saved when Byleth wrestles control from Sothis for his body.
> Byleth: You three, get away from here...now.
> Dimitri: Professor, what happened?
> Byleth: I don’t have time to explain. Just remember, if you give into your rage, bitterness will consume you. War spreads sadness and hatred, and hatred gives birth to...more... conflict... Urgh!
> Edelgard: Professor, hang in there! We’ll help you!
> Byleth: Stay away! You three...must not...end up like me! No matter what happens...do not give into rage... Do not lose sight of yourselves!
With that, Byleth ultimately escapes for their safety, and Claude tells the others not to give chase, as they really stand no chance against "her" that possesses Byleth.
Even Edelgard acknowledges and laments that the best they can do is pray, which is neat, given how she is no longer religious.
However, as they go to return, they reflect on Byleth's words.
> Claude: The professor’s words were heavy, weren’t they? Don’t give into rage, huh?
> Edelgard: Yes... But, there is always rage in the depths of my heart. I wonder... Am I in control of it?
> Dimitri: I wonder, if the one responsible for the Tragedy of Duscur appeared before me, could I contain my rage?
> Claude: Let’s return to the castle quickly! Nothing will change if you just stand there looking bewildered. It’s true though, the professor left us with some challenging thoughts. Do not give in... I must keep reminding myself of this.
What is fascinating is that this is the 3Hopes Byleth, the one who never became a teacher in Garreg Mach, and despite that, Byleth was still able to impart a valuable lesson to his. And this is arguably the one lesson that truly had all three of them shook to their very core.
No matter the universe, Byleth will always manage to be their teacher in some shape or form.