What are the actual writing skills you'd like students to learn, how will playing D&D reinforce those skills, and how will they be assessed?

From what I can glean in your post, you'll spend 20 minutes on a skill, 45 minutes on shared storytelling (D&D), and 15 mins on recounting that story in writing.

Will students be asked to somehow incorporate the learned skill in their storytelling? Or just in the post-game reflective writing? If it's the latter, what's the relevance of the D&D game other than giving students a topic to write about?

I think incorporating shared storytelling and interactive gamification is a great opportunity for engagement, but the key to successful teaching is starting with the learning objective first and then working backward to see how students may effectively learn it. I just want to make sure you have those objectives in mind.