If you don't feel like dealing with this monster (just yet) this mod allows you to completely disable it. Keep in mind that this will disable the monster permanently for any world that has this enabled (even spawning via debug menu will no longer be possible)
Download the repo and unzip the "NoShadowLieutenant" folder in <CDDA installation folder>/data/mods. It should show up under the Blacklist mod category when creating a new world.
Fair warning If you don't know what the "shadow lieutenant" is and don't want the surprise spoiled. This mod is probably not for you and you should not open the following spoiler section. The section gives an incomplete description of the "shadow lieutenant" monster's behavior (in case you didn't already know)
Show Spoilers
The current experimental version of CDDA sports a (relatively) new monster known as the "shadow lieutenant". It is tied into some game-world lore we'll not get into however.
After day 6, when you're out and about at night in fields or forests, there's a chance you might bump into this 'shadow lieutenant'. Instead of attacking you directly, this monster calls in backup, summoning groups of 'amalgamations' and shrieking to attract even more monsters to your location. Think you're ready to take it down? Think again. Dealing with this shadowy menace is no cakewalk, especially when you're surrounded by that mob of monsters it just spawned and the fact that the shadow itself tries to stay away from you as much as possible. While seasoned survivors may fare better, it's still a tough nut to crack that more often than not means the end of your precious "run". But hey, that's the charm of the apocalypse, right?