Velnias (Latvian Velns) is the Baltic chthonic trickster whom Christianisation turned into the devil: he hides under stones, in hollow trees and in the water from Perkūnas' thunderbolt, and he stands behind the folk tales of hidden treasure, contracts and shape-shifting. Comparative scholarship connects his name with the Slavic Veles and both with a Proto-Indo-European *Welnos, but the Baltic figure is documented mainly in folklore and in the devil-tales of the Christian centuries. He is not a cult deity: no temple, no priest, no offering of devotion - only avoidance, warding and the occasional bargain.
Modern Practice
Velnias is met as a thing to be outwitted: iron over the door, salt at the threshold, the bargain read twice, and the treasure that is not taken. Any work with him is protective or trickster-work, and the tradition's own posture toward him is distance.