I'm not sure if they formally joined ISIS or not, but they do show support for Al-Baghdadi. However Boko Haram is like in return for their protection ISIS essentially and want to establish a caliphate with sharia law as their legal system, along with the added goal of eliminating formal education since they see it as government propaganda.

Boko = Book; Haram = forbidden. They literally see western education as their enemy, so when they take control of villages they remove any form of knowledge and thought which just makes the indoctrination of muslims in the area much simpler. If a muslim child wasn't violent before, making them ignorant and re-educating them to believe in what they believe will surely get get the child to join their ranks. Take a look at this and see how easy it is to train children who had education: https://www.youtube.com/watch?v=wx-duzk6fhc. Imagine what they could do if free thought disappeared.

For critics and defenders of Islam(feel free to skip this): How fundamental this is to Islam has doubts and agreement. Their prophet did say "The ink of scholars is weighed on the Day of Judgement with the blood of martyrs and the ink of scholars outweighs the blood of martyrs." But it was also their second caliph Umar who said "If those books are in agreement with the Quran, we have no need of them; and if these are opposed to the Quran, destroy them" when he allegedly burned down the library of Alexandria.

Pretty much what they intend to do will be catastrophic since this is the group who kidnapped girls to sell as brides, but if you need a list of they will do:

• Issue a jizya tax on non-believers for protection(like when the mob asks for protection money).

• Kill apostates and deviants(basically any muslim who doesn't agree with them

• Execute foreigners

• Destroy schools and books that doesn't agree with them

• Stone, crucify, and behead anyone they call a sinner.

• Sell more girls as brides regarless of them paying a jizya

• Continue their expansion