What's Meruem's nen type?
According to the story, Meruem's initial ability gives him strength through consumption. His aura grows every time he devours a user of Nen, with their aura synthesizing to his own. This ability (Aura Synthesis) doesn't belong to any of the 5 basic categories, thus falls in the 6th category, making him a specialist. After absorbing large parts of Shaiapouf and Menthuthuyoupi, he was able to use their abilities too, making them his own. Further details about his new abilities can be found here.