First theory I want to share is the Stride and Al theory that I kinda came up.
As Stride was the former Sin Archbishop of Pride, and Al assumingly has Pride at the very moment, I think Stride survived the attack of the Black Dragon by using his Authority.
Living his whole life sick, I think he encountered Al once, out of anger, he used all of his power, like his Yang Sword to burn Al's face and his Meteor Rings to curse his limb, but because of this, he got weakened, and Al actually succeeded in killing him, ganing the Witch Factors of Pride.
1st off, Al wasn't even around during EX3, or in any events in general. Al entered the world around 20 years ago, and Stride was killed more than 40 years. Plus, Vetol sacrificed himself to let the worm eat both him and Stride.
And 2ndly, no one wants to know nor care about Al's backstory because it will take more than 10 years to finish his story, and the series would already been completed by then.
Edit: Plus, we don't see any relationship or whats to like about Al, he's just a nobody that has no character dev or a neuceince to the plot...
I'm not even going to respond to this message. Read my theory before responding to it.
Also, Veltol didn't sacrfice himself. He got killed by Stride during an attack on the Astrea Mansion which didn't happend in EX 3 + A lot of people care about Al's backstory, if you don't care about him, that's fine but then there's no need to respond to it.