I'm not an expert, but that looks like a heart rot, these generally only affect the heartwood, which is technically dead already, but doesn't usually rot thanks to compartmentalization.
Trees can live with heart rot for YEARS! It can and often does lead to the dimise of tree, but that doesn't mean it needs to be removed immediately, especially if the tree is not threatening a structure.
Many, many old trees conceal heart rot very well, often the wound that allows the fungus in will be high up in the branches, and goes unnoticed, I've seen many old trees that have actually survived the heart rot, all the heartwood will rot away, leaving no food for the fungus, at that point the tree will often heal itself from the inside and go right in growing.
It's also a natural cycle, the various fungi that may consume that tree will likely improve your soil, it may( or may not) cause your tree to look unattractive, but it's not doomed.
I hate to see trees chopped.
