What does "unique attack" mean in Taskmaster description?

I have just sadly pulled a Taskmaster from a Medusa featured and I have no idea what am I supposed to do with him since I just **** hate him as much Kabam hates me. Anyway I was reading his abilities and I didn't understand how the ability reduction works since I have never heard about unique attacks before. Any help?
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here is a video man
Unduped yes but once he is awakened he can reach 100% through concussion debuffs