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.

(poe) I haven't done that quest in half a decade so the details are fuzzy. Can't even remember where to find the spy. However, I think you were supposed to drop the corpse before Leto and that finished it. (kosmos) Either I have the wrong intruder or dropping the corpse (or giving it to Leto) does not trigger the quest completion anymore.

Note from Luther: I think you need the agent to meet with a Face Dancer on the way, because the dancer is the one who preserves the corpse. Giving the preserved corpse to Leto completes the quest.