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.
Imhotep will study the scroll only to learn, that two additional artifacts are needed to perform the Ceremony of Invocation, which would allow the people to communicate with the gods again. These artifacts are the Hand of Balance and the Eye of Chaos. The Hand of Balance must be retrieved in Giza, from The Great Sphinx. It is protected by the Pharaoh's Honor Guard, four large statues, which need to be defeated. The Eye of Chaos can be found in the Temple of Atum, located to the north-east of the Fayum Oasis. It is guarded by Nehebkau the Scorpion King, a Scorpos hero monster.
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.
Reward: 10000 Experience, 2 Attribute Points, random Relic Shard