So basically I believe it is about something that Kaito need to face eventually, he kinda like stuck in the past, blaming himself for parents death even though he didn't caused it, was able to survive but not his parents etc
Shiro set up trial for Kaito to face, his past, his desire etc. while this was happening, people who like Kaito got misunderstood thinking Shiro is doing something danger to Kaito. So they and Kuro declare war on god realm to save Kaito.
I think this is probably what she referred to? That Abyss is kaito's past that he always stuck. I can't remember exactly details... It's been 3-4 years ..
Fun fact:
Kaito wasn't supposed to show up now, he was meant to show up 20 years later, being undercare by different Lord (not Lilia), they don't know what to do with him, he just shut himself in his room, depression etc..Instead of Kuro who found him. It was Shiro who did, and they did make relationship similar as we saw with Kuro/Kaito.
This was future Shiro saw 10k years ago, the reason it didn't happen and it happen early is due to Kuro constantly tamper summon magic circle hoping to meet destiny