[Rias]

Spoiler for Yasu/Shannon/Beatrice:

Yasu was introduced to the family after being raised in the orphanage, and was given special privilages from Genji, since Yasu carries the blood of Beatrice (human). Other servants was jealous of that, and bullied Yasu. Yasu seeked comfort from Shannon, but it wasn't enough. Shannon persona might've been there before Yasu went to Rokkenjima, since Yasu describes Shannon as "popular in the orphanage". Regardless, Shannon persona has been there since Yasu got to Rokkenjima, if not earlier.

Eventually Yasu was engaged in magic/legend of Beatrice/mystery novels, and eventually realized that mystery - howdunit = fantasy, and became Beatrice (fantasy-wise). Yasu-Beatrice left, and Shannon persona took over, without the memories of Yasu. Years passed, Shannon fell in love with Battler, whom made a promise to return to get her the year after. However, the year after that, Asumu died, and Battler left the Ushinomiya name.

Shannon waited years after years, hoping for Battler's return, but eventually found out that Battler had forgotten about her. Heartbroken Shannon gave her feelings for Battler to Yasu-Beatrice, and Yasu-Beatrice gave Shannon a new brother: Kanon.

Shannon eventually moved on to have an relationship with George, while Yasu-Beatrice still suffered from loving Battler. After talking to Maria, Yasu-Beatrice realized that more power was needed, and went to solve the epitaph and became the "real" Beatrice. Being able to do that, Yasu was given the title of Beatrice and the as the family head (in secret, since Yasu wanted things to remain the way they were) Yasu eventually met Kinzo, forgave him of his past mistakes, and Kinzo passed away after.

The real reason of the disaster, mainly due to the trigger of the challenge to the family, isn't fully explained in ep7. We know that it has something to do with Battler's sin, of forgetting his "promise", and his return right on the 6th year. The only thing I can kind of speculate is George's proposal at the same timing of Battler's return, which seems to trigger something.