Go to Nile River West, show the Horus Flute to Huth.

Show Quest Info

Introduction

Agritt has waited a long time for someone worthy enough to help him unleash Hel's soul and acquire his power. You were his best choice. Since you have chosen to support Agritt, he says he will keep his promise. The gate to the Netherworld is actually hidden in the Pyramid of Khufu. Take the Horus Flute as an offering to begin the process of gaining access to the pyramid. You must be at least level 81 to continue the main story.

NPC Dialog

Well done! You made the right choice, not that you
had many options, haha! Now I will tell you how to
open the gate to the Netherworld. It is actually
hidden in the Pyramid of Khufu. You have to first
get the key to the pyramid. To start the process,
take this Horus' Flute to the Karnak Temple
Guardian, Huth.

Quest Completion

Hey! This is Horus' Flute! You're not supposed to
have this!... Hmm, I see, you must be an underling
of the Desert god, always opposing Horus. Get out
of here, I wont listen to you!