Which rare belt do I need to upgrade to get a Sebor's Nightmare?
Using Kanai's Cube, I can upgrade a rare item to a legendary using the correct recipe. If I want to end up with Sebor's Nightmare, do I have to start with a particular type of belt? If so, which type - they all just say "belt"?
In general, how would I know which rare item would be needed to upgrade to a chosen legendary item?
You can't upgrade rare items to a specific legendary item. What you get by upgrading is a random legendary of the same type, e.g if you upgrade a rare belt, you get a legendary belt; if you upgrade a rare helm, you get a legendary helm; if you upgrade a rare spirit stone, you get a legendary spirit stone, etc.