A man claiming to be the leader of Nigerian Islamist extremist group Boko Haram appeared in a video Wednesday taunting world leaders after military claims that he may have been killed. \"Here is Shekau, Shekau, Shekau, Shekau, original ... I want the whole world to know that I\'m alive by the grace of Allah,\" said the man, who resembled past images of Abubakar Shekau, in a video seen by AFP.