You can embed one Tabular Report into another and create two-level reports e. g. a report for Tasks, Bugs, Assigned Users, Comments into a report for User Stories using Inner Lists.
The report that is intended to be used as an Inner List should be created and saved in advance. Only already existing reports appear in the Dropdown selector in Inner Lists tab. See the article on how to create basic Tabular Reports first.
So, you need to create and save a child custom report for Tasks first (you can add filters and/or summary columns to this report as well). Let’s call this report “Open Tasks”. Then proceed with the custom report for User Stories.
Be sure to select the same process both for your User Stories and Tasks reports. Also please don't embed private reports into public ones and vice versa to avoid accessibility issues.
To embed the report, open the parent report with User Stories and go to Edit Report → Inner Lists. Select the Tasks collection and your Open Tasks report from the drop down, then click Add Inner List:
Click Save & Run Report. The User Stories will now be expandable and will include Tasks along with the columns you’ve specified for them:
Embedding of multiple Inner Lists
You can add more inner lists. For example to a report for User Stories it is possible to include Bugs, Comments, Assignments, etc.
Support for hierarchical levels and dependencies
The embedded (inner) report should fit the main report logically. For example, if you select Features for the embedded report and Tasks as top-level entities, this wouldn’t work cause User Stories intermediate level is missing.
You can only embed one inner report into a main report so far. There's no 2-level nesting.