Perkūnas (Latvian Pērkons, Prussian Perkūns) is the common Baltic thunder god, second only to Dievas: god of sky, thunder, lightning, storms, rain, fire, war, law, order, fertility, mountains and the oak. He strikes the devil (velnias) who hides under stones, in hollow trees and in the water, and the oaks split by lightning are his; his weapons are the axe and the hammer, and Thursday is his day. The sources are the 16th-17th century accounts (Łasicki, Praetorius) and the immense daina corpus rather than any temple text.
Modern Practice
Perkūnas is called in a storm and before an oath, at the oak and with iron in hand: beer, bread and the first grain, and the plain statement of what is being promised. The practice is the keeping of an order - oaths, boundaries, the law of the household - and the thunder is the witness.