Well, in the 'saga of tanya the evil ending novel', Tanya, who has been a rather ruthless character throughout, finally reaches a point where her plans start to crumble. The war-torn world she is in doesn't allow her to have complete control. There are political and military forces that are closing in on her. Her ending is not exactly what she might have hoped for as she is constantly battling against the tide of events that are mostly of her own making. She has made many enemies and they all come to haunt her in the end.
It concludes with Tanya in a difficult situation. Her actions catch up with her, and she is left to deal with the aftermath of all the chaos she has caused.
The popularity of the two can vary depending on different audiences. In some regions, the manga might be more popular because it can be easily found in bookstores and is a more familiar format for many. However, the visual novel could be more popular among hardcore fans of the series who want to explore every aspect of the story and enjoy the interactive elements. There are also fans who enjoy both formats equally.
The art styles are different. In the manga, the art is static and presented in a more traditional comic panel format. In the visual novel, there may be more detailed character expressions and background details that can be explored as you progress through the story. Also, the pacing might vary. The manga might have to condense the story more to fit within its pages, while the visual novel can take its time to develop the plot and characters.
I'm not sure specifically as it's a fanfic. It might be a fan - created story within the 'Saga of Tanya the Evil' universe that has some unique elements perhaps related to 'lemon' which could imply some sort of zesty or different take on the original story.
A common theme is survival. Given the dangerous world Tanya is in, many fanfictions focus on how she continues to survive in various difficult situations. Another theme is power struggle, as she has to navigate through different power hierarchies.