Lava
It's a substance that is underground.
It can be incredibly destructive.
It ignites most of what it touches.
Rather than 'ignite it' the villain evokes or triggers a lava flow or an eruption.
Beyond the infamous Pompeii eruption in the Roman era, Mount Vesuvius erupted after WW II and destroyed some local towns around Naples. (My source on that is a book Naples '44 by Norman Lewis that I no longer have access to).
There is ongoing lava flow from Etna, which is on Sicily, in the news now and again.
Since I also play D&D 5e, I'll offer to you that your villain can get access to the level 8 earthquake spell (druid/cleric) and use that to create deep enough fissures, or a fracture/fissure in just the right place, to trigger a locally risky fault zone that either starts oozing lava, or erupts, based on how you want to narratively implement the Lava hazard. Do you want slow and inexorable, or fast and furious? That's up to you as the DM.
You can also, as the DM, establish that the villain is the only one who knows how close to the surface the lava zone is, such that the calamity comes as a surprise to the local inhabitants. You have a lot of latitude in the 'how' of implementing a natural disaster.
Examples close to your time frame: Monte Nuovo or Vesuvius
I used to live in the Campi Flegrei region of Italy.
Monte Nuovo ("New Mountain") is a cinder cone volcano within the Campi Flegrei caldera, near Naples, southern Italy. A series of damaging earthquakes and changes in land elevation preceded its only eruption, during the most recent part of the Holocene, which lasted from September 29 to October 6, 1538, when it was formed.2 The event is important in the history of science because it was the first eruption in modern times to be described by a large number of witnesses.[3] The eruptive vent formed next to the medieval village of Tripergole on the shores of the then-much larger Lake Lucrino. The thermal bath village, which had been inhabited since ancient Roman times and was home to notable Roman-era buildings including Cicero's villa, was completely buried by ejecta from the new cinder cone. Tripergole's ruins and its important thermal springs completely disappeared under Monte Nuovo such that the exact location of the village can no longer be identified.[4]
And then there's Vesuvius again...
In 1631, Mt. Vesuvius gave vent to a powerful eruption. By all accounts, it was a highly explosive event that rivalled in intensity the famous eruption that doomed Pompeii and Herculaneum in the first century a.d. Sources say that the eruption destroyed most of the towns in the area of Vesuvius. The event was so terrifying that it stoked the creative imaginations of the great painters of the day, primarily > Micco Spadaro (name in art of Domenico Gargiulo, 1610-75). His "Eruption of Vesuvius in 1631" (painting, right) shows the procession of the populace, viceroy, church prelates and aristocracy. They carry the bust of the Patron Saint, Gennaro, in a show of penitence, invoking divine mercy.
Given that this is set in D&D 5e: magic exists.