The size of the Nirvana Flame Divine Cauldron could be controlled, this cauldron could turn into a room to be so big most greatly.
It could be used for multipurpose, for alchemy, artifacts refining, also for fighting, but used to defend is most appropriate because of its hardness.
It can be used as storage equipment, space can hold a mountain.
Feng Yun's mostly attracted by the artifacts refining, his demonic flame was an ideal flame for refining artifacts or pills, but he was interested in the artifacts. A spiritual weapon could increase the strength of any cultivator many times. Making him undefeatable in the same realm.