Solution

Arrakis AP: go to Onn (35,25).Take 100k solaris with you before you start. The 100k is to purchase an ornithopter if you don't already own one.

This quest starts in the Onn on Arrakis.

From the AP go e,29s,enter and find Leto, listen to what he says.

Leto says : There is an intruder in this place. I want her dead! be sure that the corpse has been preserved for me to show the world what will become of traitors of her kind!

Find the “Fish Speaker Undercover Agent” and say

          "help me\ntrack intruder". 

The agent will lead you to the intruder, on the second floor of the Onn:

This man is very large and has a guttural tone to his voice. There is an odd style to his clothes and hair, as well, hinting that he may be from outside the realm of the Emperor. A symbol along the side of his pant leg shows he comes from the planet Gammu, or at least stopped there before coming to Arrakis. The phoenix printed on his collar is the sign of an Honored Matre.

     
      You have to find:
     * A Face Dancer
     * A Weapons Merchant
     * An Undercover Agent

     Face Dancer and Merchant can be found on Plaza or on walls. 
     Agent is on the top of one of the north east tower.

When you find each of them

     'say Could you help me' 

and after this:

• 1 to Dancer : say preserve corpse
• 2 to Merchant (worked with Weapons Merchant): identify matre
• 3 to Agent : find spy

Speak to agent at the very end cuz he'll lead you to they spy you have to kill. When he finds matre, listen to what your companions are saying,

Turn off any triggers you have that do stuff with corpses before you kill it.

After dancer preserves corpse, take it and give it to Leto (he's on plaza, max n from entrance).