Skeletons of Dhuum are the first minions Dhuum summoned as his power grew again in the Underworld. Attacking with projectiles made of their own enchanted bone, they are able to penetrate any and all defenses their foes bring to bear.
Both of their skills are able to "penetrate all defenses", meaning that they always hit (although Bone Spike can be dodged) and they always deal full damage, regardless of any damage mitigation effects on the target. A number of key skills in popular Underworld farming builds are thus useless against these two skills, including Shadow Form (for the permasin) and Protective Spirit and Shield of Absorption (for the 600 Monk and the invincimonk).
Their normal attacks, however, do not have this ability and are still affected by the above skills.