I have an extremely sore heel, not the sole (plantar fasciitis) or achilles, but the actual heel bone. What could this be?

Heel pain. There are a number of reasons for heel pain. Most common being plantar facsiitis. If it is the calcaneous (heel bone) it may be a stress fracture or a bone bruise. There are some arthritic conditions that can cause this problem. Best to have it checked out by your local podiatrist or orthopedic surgeon.