The plot involves Rimuru's journey from being a rather weak (initially) slime to becoming a very powerful being. He meets many interesting characters. For example, there are the goblins he befriends early on. Rimuru helps them improve their living conditions. As the story progresses, he also has to deal with the threat of other more powerful monsters and even some human kingdoms that view his growing power with suspicion. In general, it's a story full of adventure, growth, and the forging of relationships in a magical and dangerous world.
It might start with the main character noticing some odd things on Tumblr related to Rex Testarossa. As they dig deeper, they start to experience some spooky events. For example, they could receive anonymous and menacing messages, or they might see disturbing images that seem to be connected to Rex Testarossa. The plot could then build up to a climax where the character is in real danger from whatever dark force is associated with Rex Testarossa.
It could also be the psychological aspect. The story might play with the characters' and readers' minds. It could make the characters question their own sanity, like seeing things that might not be there or having memories that don't seem to be their own. This psychological confusion can be very scary for the readers as they start to question things along with the characters.
The main character is Rimuru Tempest. He is a slime who has unique powers and a great deal of charisma. Another important character is Veldora, the storm dragon who befriends Rimuru. There is also Shizu, a human who has an impact on Rimuru's early development. And then there are characters like Benimaru, a powerful ogre who becomes one of Rimuru's loyal followers.