By all indications, Dr. Naif Al-Mutawa's comic The 99 is a huge success. It has even inspired an animated series and a series of theme parks. But the series and its creator have also run into some trouble.

In addition to earning criticism from American conservatives, the series has been called blasphemous by some Islamic clerics. They say the comic's core concept of 99 young heroes who reflect the 99 attributes ascribed to Allah in the Quran mocks the Islamic religion. Now, the Iraqi militant group ISIS, which has been systematically taking over cities in the country, is calling for Mutawa to be killed, and has even offered a reward.

According to the Kuwait Times, one ISIS Twitter account tweeted, "Who can kill [Naif] Al- Mutawa who makes fun of Allah’s names?” Another said, “...whoever finds him, kill him, and he will be rewarded.” Yet another tweeted said, "There is no good in us if he remains alive for over three days." An al-Qaeda account agreed with the sentiments.

Mutawa doesn't seem to be backing down. "My work has glorified Islam from the US to China for the past ten years," he tweeted in reply. "I really do not believe in [ISIS] and Qaeda … I don’t care about them."

There's a good chance Mutawa expected a response like this to his comic. He has previously said the intention behind the book was to "take back Islam" from militants and jihadis.