That was his Class Skill 'Frost Strike' of the Frost Lord after his career transition, which, when stacked with the Combat Skills Shield Bash, also greatly diminished the Frost Damage.
This also explains why, when he fought that assassin last night, despite hitting the opponent twice consecutively with Shield Bash, he was unable to extend Frost Power through.
Of course, this wasn't a problem with the Obsidian Defender, but rather a difference in circumstances; when he initially acquired the Obsidian Defender, he hadn't transitioned, nor could he have known in advance that he would obtain such a Class Skill as Frost Strike.
Moreover, when facing casters or creatures like Ice Spirits, the Obsidian Defender is still effective.
However, similarly, when his Frost Power passes through the Shield, it also gets weakened.
