The Demon Crystallizer can be crafted within the Hellfire Forge. Once you have set up the Demon Crucible you can put down a Demon Crystallizer in the same chunk as the Crucible. The Crystallizer will slowly draw out the Will in the Aura and will form them into a Crystal. These crystals will appear as one big crystal and slowly get bigger and bigger finally forming a crystal with 7 spines. These can then be broken off (with a pick axe) and collected, but don’t do that! There is an easier way to collect these crystals, you're better off not smashing the cluster with a Pickaxe. Instead, if your inventory is holding at least 1000 points' worth of Will (in Tartaric Gems), then you can harvest a cluster by right-clicking with an empty hand. You will get the up-to 7 "secondary" crystals but leave the "senior" (straight upright) crystal intact. So you can keep harvesting crystals from a single cluster so long as you keep Crucible-infusing Will into your chunk.
Because the central upright crystal takes 90-100 Will from the Aura to form, it’s significantly more expensive than the secondary spires, which normally cost 50 Will. There are reasons that you may need to break the central spire, but only do so when you’re ready.
These crystals are normal will crystals they can be used for various crafting recipes and it is highly recommended that you set up a few of these Crystallizers in order to farm up as many of these crystals as possible.