Well, spoilers for a sequel could involve the villainess facing unexpected rivals or having to deal with the consequences of her past actions in more complex ways. But it's best to discover these as you read.
The biggest spoiler of 'A Beast Tamed by Villainess' is the revelation of the protagonist's true identity. It completely alters the way you view the story. Also, there's a surprising turn of events in the final chapters that you wouldn't expect. However, it's best to discover these while reading the book.
Well, spoilers in light novels related to gamers can be really annoying. For example, if you're into a game - based light novel series and someone reveals a major plot twist about a character's in - game power - up or a crucial event in the game world that the characters are part of. It can ruin the excitement of discovery when you finally get to that part of the novel.
Well, there are some major plot turns in 'A Will Eternal' that you wouldn't expect. However, I suggest you discover them yourself while reading to fully enjoy the story.
A common trait is also being manipulative. The villainess will try to manipulate the situations and the people around her to get what she wants. She might manipulate the male leads by lying about the protagonist or creating misunderstandings between them to drive a wedge in their relationship.
Yes, gamers can be affected by spoilers in light novels. If a light novel is based on a game they love, learning key plot points in advance can take away the joy of experiencing the story as it unfolds in the book.