I would say, “they plotted against him, poisoned him.” You want to keep the verbs the same tense. You could also say, “they plotted against him, poisoning him” But, the meaning is a little weird. Did they give him poison so he would die? Or are the poisoning him with their words? (Lying to him, making him feel doubts, etc) What’s the meaning you want?

Hi! Yes, that can used in a literary work but it's not common everyday English. It sounds like it's from a book or maybe a theatrical play. When it comes to how some writers script books, they may write in ways and terms that don't follow everyday grammar rules. However, they are considered correct most of the time. Just curious, is there more to the sentence? Where did you read that?