For those who have not completed games in the Xenoblade Chronicles series, this page contains spoilers regarding the plot. Discretion is advised.
Guldos are a family of enemies in Xenoblade Chronicles 2. They are dark gray humanoids with glowing blue growths found only in the Land of Morytha. Guldos come in three varieties; small blueish ones, larger reddish ones and gargantuan blueish ones.
Guldos were once humans who tried to replace their brain cells with Core Crystals in an attempt at immortality after the world was left in ruins. The experiment had mixed results; they were corrupted and transformed into horrible mockeries of their own selves, but they successfully managed to survive for millennia, obtaining healing capabilities similar to a Blade.