When I install abyssalcraft it keeps saying it required the accore core to be installed. If this is something that has to be installed can someone give me a link.

Did you do anything to the MANIFEST.MF file inside of META-INF? And where did you download the mod?

No I did nothing to the Manifest.MF and I believe I downloaded it from curse

I will gt back to you with more information on it tomorrow because it may have been from a past update. and I need to update to the correct forge but this has happened before. I may be mistaken about this in the most recent update of the mod.