Inevitability is the fact that something is going to happen, and nothing can stop it. You use it when the ending already feels fixed: a storm that will arrive, a change that will come, a secret that will come out. People also use it for the thing itself — “War began to feel like an inevitability” means war felt like the thing that was bound to happen, not just a chance. It is a rather formal, bookish noun. In ordinary talk, people more often say “it was bound to happen” or “there was no way around it.” Writers like it because it names the heavy feeling that the future has already been decided. Though actually, speakers sometimes reach for it when they only mean “very likely,” which is a little looser than the tidy dictionary picture.