Before you can enter Himmelmez's Chamber, you would need to complete Dark Whisper quests and gain access to Silent Corridor and Assassin's Sanctum.

To start the new quest line, locate Abdullah in Morroc City. The first quest will be titled, "Assassin's Duel".

Silent Corridor 

To enter Silent Corridor, head to the portal marked on the map.

Once inside, speak with NPC April to start the challenges. 

There will be 5 rounds in this dungeon, and you will encounter the following bosses:

Round 1: Dona

Round 2: Raphi

Round 3: Reo

Round 4: Miki

Round 5: Dona, Raphi, Reo, and Miki

Assassin's Sanctum

The entrance to Assassin's Sanctum will be located in the red area marked on the map.

In this dungeon, you will encounter the following bosses:

Assassin Elder

Francis Accel Valatie Jeff

Eremes

Himmelmez's Chamber

Before you can enter Himmelmez's Chamber, you would need to complete the Dark Whisper questline which requires you to acquire 8 floating stones. Find Dwayne to start the questline.

Once you acquire the 8 floating stones from the bosses in Silent Corridor and Assassin's Sanctum, head to portal circled in the map above to gain entrance into Himmelmez's Chamber.

Himmelmez and her Dragon