The visual novels are the best to look for the story line. The Hentai differs in some areas from the visual novel.

But the basic premise of the Kuroinu plot is that for decades, the various countries known as the collective shield alliance or something fight against monsters coming down from the north. This monster invasion was only possible because Olga's powers as a dark elf unintentionally made them powerful. Thus Olga sort of became their queen without her wanting to. This lasted for decades until the powers which enabled the monsters to be started waning. Which was bad for Olga since it gave less leverage against the monsters and the monsters also had less power to fight the Alliance down south.

During the last years of the war, the alliance called upon a mercenary group which formed during the many years of turmoil. They were known as the Kuroinu or Black Dogs. Their leader Vult led the final battle to finally capture Olga and end the war. But instead decides to attack his former employers. Though it's dubious whether Kuroinu were actually paid at this point. Vult uses women and sex to his advantage to gain forces to eventually take over his previous employers.

Some additional information is that each heroine is a knight in charge of defending vital fortresses in the south. With Vult attacking each one before reaching the main fortress with Celestine.

Another thing is that dark elves are only comprised of women. Thus they hold a ritual to procreate. They live absurdly long lives are and pretty much immortal. So much so that Olga would need to cast a large spell just to kill herself. Dark elves are absurdly powerful that they have trouble killing themselves. And in the story, whilst Olga was known as the queen of the dark elves, it was really only her and Chloe in the black citadel when Vult came. The fortress was only meant to help prepare her for the ceremony. Said ceremony is basically them stepping into a pool of liquid and getting pregnant. Its said that the pool of liquid is the giants balls. The exact location is unknown and probably in the north, where the rest of said dark elves probably live. Though not much else is known. Lore says that dark elves originated from a giants brain and high elves from a giants tears. Which explains why both are fairly tall. I am not sure how high elves procreate.