I'm not entirely sure as the description is rather unclear. It could potentially be about a character named Issei in the 'DxD' universe and some sort of action related to a fanfiction about him. Maybe it's about exploring or creating a fan - made story where Issei is involved in a stroking action, though without more context it's hard to be specific.
It could be his fellow team members. They might betray him out of greed for power. Maybe they thought that by getting rid of Issei, they could gain more resources or a higher position within their group.
Issei could react with anger. He might lash out at his partner and the person they cheated with. He could also become very confrontational, demanding explanations and apologies.
Rias could be a main character. She has a strong connection with Issei and in a fanfiction about him as the Prince of Darkness, she might play a role in helping him control his new power or deal with the political ramifications of his new status.
Issei could develop by learning the discipline and self - control that comes with being an exorcist. This would contrast with his more impulsive devil - side nature. For example, he might have to resist the urge to use brute force and instead use the more strategic and spiritual techniques of an exorcist.
In a cultural context, if 'el vate bases on el dasa story', it could mean the preservation of certain cultural values. 'El dasa story' might be a traditional story that contains important cultural elements. 'El vate' then becomes a way to carry on these elements. For example, if 'el dasa story' is about family loyalty, 'el vate' could promote the same value in a different form.
A scary El Salvador story is that of the Xtabay. It's said to be a female spirit that lives in the jungle. She sings a beautiful but hypnotic song that draws men to her. Once they are close, she either kills them or drives them insane. This story was often told to keep people, especially men, from wandering too far into the dangerous and uncharted parts of the jungle, as the unknown held such terrifying entities.