First proposed by Paul Birch, a Birch Planet is an extremely large shellworld which is built around a supermassive black hole as a habitat.
The largest possible size of a birch planet is one built around a black hole with an event horizon with equal surface gravity to Earth. Such a black hole has a mass 1500 billion solar masses (the mass of an entire galaxy) and a diameter of 1 light year. Beings living on a birch planet experience the pronounced effects of time dilation. However, due to the extreme mass of the object, tidal forces would be extremely small, meaning many thousands of shellworld layers can be built before loss of gravity becomes an issue. Because a black hole takes an incredibly long time to burn out, these could be the last homes of civilizations in the universe.
Several exist within the universe, but these are few and far between. One benefit to living on one, however, is that the time dilation is so great, that any entity trying to attack or even communicate with such a place would have to wait millions of years before knowing if their invasion were successful. Since few civilizations have demonstrated living for that long, the high cost of invasion, and the risk of stranding one's troops millions of years into the future are all deterring factors. A downside is that they have no way to communicate with any outside civilization. This is circumvented by simply sending representatives outside of the planet to experience normal time before re-entering the Birch Planet to report on their findings.