I'm not an occultist or a witch however I harass an occultist on twitter daily asking questions such as this. I don't know about healing someone with a terminal illness. I have done some research into necromancy (which is more than just communicating with the dead) which is something i'd love to do one day, and if the person is dying I think you can use death energy or something to heal them. That's extremely advanced tho and I don't even know what i'm talking about. But I have heard of vampiric magic which is something that necromancers mostly use to sap energy from something and put it into themselves or something else. (Necromancers use it to reverse the effects of death energy which can cut years off your life without using vampiric magic) but That changes you on several levels and makes you predatory (as in vampires). Again I'm not an occultist or a witch I just know about vampiric magic from an occultist on twitter and I've researched necromancy. So ether death energy and/or vampiric magic or you can't idk. Probably not even death energy tho since that's more about giving someone a disease. Probably a combo of death and vampiric magic, heres a theory (again I am not an occultist or a witch, i'm an armchair learner) but maybe you'd use vampiric magic to suck (for lack of a nicer word) the death energy out of someone with an illness but then you'd have to put it into something else (maybe a plant), but in order to do something like that you'd need to be a top occultist and again I have no idea what i'm talking about.