I just remembered that I had something to say about the 7th. I'm laying here in the dark trying to get some sleep and my tired brain kept bugging me until I remembered I had to post this. Anyway, Dani might actually be apart of the 7th's line. That's the take I thought of yesterday.
We learned a while ago that during evolution, you can potentially see the progenitor of the entire line during the process. People under the 8th said they saw it, even though they aren't direct descendents like Tiffany. Alice saw the 6th during her evolution as well. So with Danica's 3rd evolution, which was a natural one, she likely saw an Apostle for the first time. From her retelling, it was a much horrible experience than the previous.
Now, here's why I think that she had to have saw an Apostle. The M65-Gestas wasn't that powerful when the MC(Level 2) saw it in his dream. Sure, he had a reaction to it but it wasn't as bad as when he saw an Apostle. So it's maybe a B-Rank monster. Yet, Danica reaching Level 3 said it was an awful experience, when she's basically equal with it now. She would've already saw it during her 2nd Evolution, so why would she be bothered by it on her 3rd? That's what led me to believe she saw an Apostle this time.
Here's my speculation for why it's the 7th. While Dani's powers are called "Cross Light" her X Marks the Spot ability is actually black. Her papa, Gestas, black cross resembles the same energy that Valravn gives off. Then there's the important fact that Danica is on the kill list of the Arbiter/Eye Boy. That was always odd. Jake, Kenny and Bailey I get because they're all related to the 1st, mostly. Yet, Danica was an odd one to mention.
With her power name, it could be assumed that she's mostly from the 8th's line, among other minor combinations, but it's possible that the 7th can pass on the Light trait itself to it's progeny. Since we already know it's possible to cross inherit the twin traits and that the 7th likely snuffed out his twin or put them in a deep sleep. It's possible that he spawned monsters that inherited light or went around corrupting the 8th's line with his darkness, altering them. It would explain why Gestas looks like it's pinned to the black cross and why it's covered in black. Something that Dani also noted while describing it. That it seemed to possibly be done by another monster.
The process of elimination that led to it being our Dark Lord over the other Apostles. From all of the evolutions we know about, the common theme is that it's either a comforting feeling or dreadful. Jake(fear) and Deryl(failed) showed the dreadful side while MC, Alice and Michael all said they felt blissful. We can then eliminate 2nd, 3rd, 4th and 6th. Her power is too distinct to be of those lines. The 1st and possibly the 5th are eliminated as well. That only leaves 7th - 12th. 8th would be the most likely candidate but they're missing, and the other twins after are implied to be dead or missing as well. So she likely didn't see them. That means the 7th is the only one left.
The visuals imply that Gestas is of the 7th's line or warped in some way but Dani's shown power and name implies it's apart of the 8th's. It's called Cross Light but they're formed from her black tattoos and her strongest ability forms as a black X. If she truly saw an Apostle, the only one available that lines up with her power is the 7th. It would also explain why it was an awful experience for her. MC just dreaming of the 7th terrified him. If Dani actually saw him appear during her evolution, it would undoubtedly be a horrible experience.
Anyway, that's my early morning theory. The idea hit me a few days back while speculating about what powers Dani could gain and actually thinking about where it could go. By the way, this is pure speculation. It's possible that the Gestas is much powerful than I or the MC gave it credit for. We also don't know anything about the 9th Apostle and beyond so if she did saw one, there's nothing to say it wasn't one of them. This also hinges on her seeing one in the first place. Maybe she just thought the evolution was just the worse because she could remember parts of it now on her 3rd Evolution.