If you want to enter nirvana, you must absorb enough nirvana and nirvana. There are two ways to get it. One is to take it from heaven and earth, which does not require nirvana Dan, but the efficiency is not high. The other is to rely on foreign things to refine something containing nirvana, and the best substitute for this kind of thing is nirvana Dan.
However, Nirvana Dan is very precious, even if it is strong in Nirvana, it is