The sister sigil to the Water Sigil. Press [ПКМ] when holding the Lava Sigil to create a source block of lava on the ground, for the cost of 1000 LP. Crafted in an Alchemy Array using a Lava Reagent and a Blank Slate, it'll drain 5 hearts from you if you don't have enough LP in your Soul Network.
Lava Sigil Array
The Lava Sigil, next to its crafting array, plus its primary use.
What's more, it can be used in the Alchemy Table to automate the production of Lava Buckets. The Sigil is not consumed in this recipe.