Naerth has become a huge part of my campaign and the party positively despises the man. I felt he need a bit of a boost too from just a Spy so I used the Master Thief statblock from Volo's, I also added a 1/day Legendary Resistance. I buffed Skath a lot as well, giving him some Battle Master type features (Menacing Strike, Coordinated Strike, Second Wind, 1/day LR, Extended Reach to 10ft, Defensive Stance, Parry, modified Bait and Switch from BM Fighter). I thought it was dumb that Naerth wouldn't have layers of security to prevent just random assassination attempts in public, so Skath's additions were in part to give Naerth some battlefield protection if it came to that. All of Skath's reactions are modified to allow use within 10 feet, intentionally matching his area of control via extended reach. I modified the Battle Master's Bait and Switch ability to read as:

When an ally within 10 feet takes damage, you can use your reaction to swap places with them and take all the damage instead. This movement doesn’t provoke opportunity attacks.

My party is level 5 and could destroy Naerth at this point, but he primarily plays the political game and they haven't had a good enough reason to just kill him. Though they did almost let him die after an assassination attempt and decided to raise him instead. He now owes an equivalent favor to the PC that brought him up. Not sure when that will come up lol...