The Invocation is the 9th chapter of the main quest, given and completed in Egypt (Act II) and following The Blindness of the Gods. The chapter starts when you bring the Scroll of Invocation (obtained from the Rhakotis Library) to Imhotep in Memphis. Return with both artifacts in your inventory to Imhotep in Memphis (returning with just one artifact will make him tell you to go find the other one). Imhotep will try to perform the Ceremony of Invocation but fail. However, the Gate to Thebes will open, completing this chapter. The next chapter starts, A Telkine in Egypt.