Return to the Temple of Elemental Evil offers an epic adventure that takes characters from 4th to 14th levels. Caves of Ancient Secrets contains additional encounters designed to be played with that adventure. This small section of caverns, older than The Temple of All-Consumption (see Part 2 of Return to the Temple) is played during Chapter 5: The Crater Ridge Mines. It climaxes in a meeting with the half-fiend aboleth who took over the role of the Master from the current Second of the Triad (see Chapter 6: The Inner and Outer Fanes).
The bonus material in the Caves of Ancient Secrets web enhancement adds to the background and theme of the entire Return to the Temple of Elemental Evil adventure. It’s exclusive to the Wizards of the Coast website.