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Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What's the whole deal with The Satanic Verses and what in it caused a fatwa on Salman Rushdie's life?
gpt
Some Muslim scholars, philosophers and biographers tell a story called the Gharaniq Incident. Basically what happened is that Mohammed, when he was alive, set about converting the Mecca area to his new religion (Islam). People in the Mecca area worshipped Lat, Uzza and Manat at the time. One day, Mohammed was reciting words he believed to be delivered by the angel called Gabriel, when Satan entered his mind and added extra verses that implied Lat, Uzza and Manat actually existed, and can be prayed to, which Mohammd speaks aloud and commits to scripture, believing Satan to be Gabriel (which obviously hurts his attempts to convert them to a different religion). The angel Gabriel is disappointed in Mohammed, Mohammed takes back what he said. The verses Satan put in Mohammed's mind are the "satanic verses".

Now, this incident isn't actually in the Qur'an, it's an apocryphal tale, and most Muslims reject that it ever happened. You can see why it would be controversial.

Rushdie's book involves Gabriel and Mohammed as characters, and retells this incident. That's controversial enough, but many Muslim critics claim that Rushdie depicts Mohammed as too flawed, and at one point, Mohammed's friend says that he believes Mohammed is editing the Qur'an as the angels dictate it to him, to suit his own beliefs and purposes. 

There's another segment in which a girl believes that Gabriel is speaking to her, and telling her to make a pilgrimage to Mecca across the sea. She and her flock will be allowed to walk on the surface of the water. She gathers her supporters and goes, but they walk into the water and vanish, with it left vague whether they actually walked on the sea (because she was really speaking to angels) or whether she drowned (because she wasn't). 

The combination of these things was enough for it to be deemed incredibly blasphemous. Ayatollah Khomeini issued an order for all faithful Muslims to kill the author, Salman Rushdie, if they got the chance. Salman's still alive, but many other people were killed; bookstores were bombed, translators were murdered, and several Muslim leaders who opposed the decision were murdered by their congregations.

Rushdie was called on by Muslim leaders and some politicians in Britain to disown the book and cease publication. He issued an apology that pretty much said "sorry it offended you", but didn't disown the book or stop publishing it. There was a big debate between "We should have freedom of speech, and shouldn't cave in to violence or threat" and "You should stop offending people, it's causing problems and is disrespectful".

There was a similar controversy surrounding the novel and Scorsese film "The Last Temptation of Christ". The core idea of that novel is that Jesus of Nazareth was both fully divine and fully human, and as a human, experienced human feelings like anxiety, emotional and sexual desire, fear, and reluctance, which upset many people for many reasons. The author of the novel received death threats, and some Christian protest groups burnt down cinemas who showed the film and assaulted people who bought tickets. That incident is worth reading about because it was more localised and lacks the international-politics angle.