The photos are not distinct enough for me to state exactly what is going on but it is possible there is heartwood rot or root rot.  You need to have a certified arborist make a house call and diagnosis.  It is possible that nothing can be done but if it is rot, the tree could quickly become a hazard which could fall on people/property during a storm.  Go to this link and click on Find An Arborist.  Put in your zip code when prompted and a list of certified arborists in your area will appear:  https://www.treesaregood.org/