Word Ending huh, well, world ending is not neccessary to be Planet level, someone with AP from Country level to Multi Continent could "ending the world" over time, like well, nuclear bomb also have power to ending the world you know.
I admit my memories with HG2 is really foggy, but HG2 Kiana is not Sirin Reincarnated, 2nd Honkai Errupt happen similar to HI3 and her consciousness reside in her gems after her physical body being destroyed, it stored in the Moonlight Throne battleship, thing happen here and there, well Senban Highschool and the whole zombie thing i don't want to repeat, it is freaking long...wee because of Cocolia doing her stupid thing, the core fused with Kian and turn her into Herrscher which later called HotE. She is called HotE because she is the end - the last herrscher appeared and end humanity, that all, and HoT Mei of that time is freaking weak. That is the end of the main story. but yeah the game still living so more content out with DLC, but most of them are branching timeline so we all kind of new char like Delta, Jhana who appear in Era:Zero DLC - the DLC which being cancelled. There are more, but shit this is so freaking headache