[@Regitnui]
Anything stored by Void is 'locked' in time. It doesn't age or act. So if she stored a candle, it wouldn't burn down even if she waited for a hundred years. In addition, momentum and state are maintained. So something that's burning when she stores it will still be on fire when she summons it again.

Combining these two factors, it's possible for her to store a natural phenomenon such as a 'fire whirl', and it will maintain both it's momentum and heat. Of course she only needs to 'store' the 'bottom' part, as the phenomenon will automatically expand upwards if provided with sufficient fuel.

As to where she got this thing.... well, they do occur naturally on occasion. And it would be possible for her to simulate the necessary phenomena to cause a fire tornado to appear, although this would take a while. So this particular fire tornado may be a naturally occurring fire tornado she caught, or one she built herself and then stored.