Oh, you are right, the Task in planner:
You first create the task using the following block:

Title and designated users can be produced from the dynamic data from the message, but this does not have the description field.
For that you need to use the "Update Task Details_V2" block:

In this case, I used the message subject as task title, and the message body (in plan text) as task description.
As you can see, you can even create or modify checklists and checklist items, link to existing files, etc...
Careful: When you add this block, the ID field tries to list all possible available tasks, but at the end of the list you have the possibility to "Enter manually". If you do so, you can get the ID of the freshly created task from the dynamic content.

So, the total flow is like this, in blocks:
