If a User Story is very complex and requires a lot a work, you can consider that User Story an Epic. After you break down the complex User Story into smaller User Stories (which of course you can break down further into Tasks), you can group the resulting User Stories under that same Epic.
Under the same Epic you can even add User Stories from another Taiga Project.
A User Story can belong to one or more Epics!
Working with Epics gives you another perspective of how work on a product is progressing. Even if working on that product requires bits and pieces that have already been done or are in progress on other Projects, it will be easy to view the overall status of work progression.
When you access the Epics Dashboard, you can view a progress bar for each Epic. As you close User Stories from an Epic, you will see how the progress bar moves towards 100% completion.
After you create a new Epic, you can access its details page where you can manage and edit its settings if necessary.