Shadows are cloaked assasins that will climb walls and go straight for your defenses. They cannot be detected by turrets and must be in vicinity of a motion sensor and lit up by a light source(unless the turret has a Detection Mod) to be targeted.
It appears to be a basic Goblock wearing a full set of black cloak and hood. Shadows also weild a black sword for destruction.
Since turrets without the proper modifications and help ignore these Goblocks, it is best to maximize their detection by implementing motion sensors and ensure that the area around the placed motion sensors are well-lit.
It is recommended that you keep a look out for these guys and watch your back as they can slip in and deal too much damage to your motion sensors before your turrets have a chance to target them. This is especially important when Shadows come in as a group, together with their leader. Use weaponry, such as the Sniper Rifle, to destroy them before they reach your base.
You can also bait them with POWERED light blocks:
Firstly,, dig 2 blocks underground. Then, build a motion sensor, followed by a light block on top. Ensure that your turrets are in range in order to kill them. Note that if the light blocks are not powered, they will ignore them if a better target, such as a Power Block, is in range.