It was the perfect opportunity for him and one not to be missed especially when the Underground despite its reputation has proven time and time again of its usefulness to the deserter; that was even when it was mostly blocked off as a result of the attacks, contained many hazards in the form of Padfoot's Freakshows and, had many other spots closed off to him for various reasons. Now cleared and open to those who volunteered to contribute to its reconstruction whether or not they had any intention to like F'mos himself, the Aukari was able to set his plans into motion which would be a boon to his quality of life in the city he now called home.
Obviously with his kin responsible for some attacks on Etzos just recently, F'mos knew to get caught would be disastrous and would they even trust an Aukari like him with the construction to begin with? Fortunately he had a means to get through that obstacle although he was not too sure of its effectiveness as it was always a toss up for him to make use of it. He needed something else, something which he could use to reinforce his cover as he explored the Underground for himself. With the items he believed were necessary for his plans and given the demeanour of the one person he knew could supply him with it, it was inevitable that he would eventually drag Nightshade Eld into it.
One thing the Avriel would be able to supply him or so he thought, was a detailed bird's eye view map of Etzos which he could use to pick out the places crucial to his plans- at least it should after he had a map of the Underground for himself to use with in tandem. As the changed layout of the underground structure necessitated a much more current map of the Underground, one which Nightshade could not get even from her place of employment, a fresh exploration of it was tantamount and someone incapable of mapping like F'mos could only look to Nightshade for assistance.
Which led to the second advantage of Nightshade being with him and during their exploration of the Underground. Alone and without a partner of his own for obvious reasons, even in his Black Guard disguise F'mos might be questioned or prevented access from certain places entirely. With one of the many heroes present in Etzos on the other hand, one which was renowned for her tendency to help the people unlike the reputation held by Doran as one specialized in harm, F'mos could just as easily say he was one of the many Black Guards tasked to serve and protect those volunteers who risked themselves in the reconstruction effort. That or the Guard tasked to accompany the Avriel during her visit. It did not matter to him which excuse he used as long as it worked.
F'mos deliberately led Nightshade, or if the Avriel proved too headstrong for that, directed her through the maze for as long as she was willing. There were places he knew he would be able to enter on his own and map out if he needed; places which belonged to the organization he worked with in Etzos which the hero did not need to know about because undoubtedly she would get into its business. The other groups which used the Underground for its purposes including the Black Guard which the Aukari would not be able to enter on his own under normal circumstances and especially, the groups which actively opposed or troubled the Fence which F'mos knew of though... the places under their control were the focus of the Aukari that trial.
If the hero decided to get up into those groups business after their exploration, F'mos would consider that as a bonus to claim credit from with the organization.
Until then, he needed to make sure he succeeded in his goals. It went well for quite a bit, the legitimate Black Guards were too busy maintaining order for all the volunteers to bother with him and the other people who may take issue with a random person wandering the Underground were much too tense to make sure he was an actual Black Guard. Unfortunately their progress was finally stalled by the one group after all the others which did not seem to know better as they started to harass Nightshade despite the Blackguard which accompanied her. F'mos tried to step in but it seemed to only escalate things the moment he did.
"We don't take kindly to your kind here" they said, which was the reason why F'mos initially thought their issue was with Nightshade but their response to him that "the flapper is with me" in an attempt to win the racist bigots over proved otherwise; especially when it was decidedly pointed at him "no, you." Not too interested in starting any trouble especially when there were more important things, at least in his opinion, to attend to while the opportunity was available to him, F'mos tried to quickly get away with a "we're just passing through citizens" as he headed behind Nightshade as both protection and to rush her through.
He had no idea what the heroic Nightshade Eld would do with these ne'er do wells though, but he could only hope it would not turn out as anything too difficult for either of them.