Triglav was a pre-Christian Slavic deity worshiped especially in the area around the town of Stettin. In Stettin itself, the statue of Triglav sat atop the center of three hills. The golden idol had three heads, and its eyes and lips were both covered by golden veils.
The name Triglav itself means "three heads," an aspect of Triglav that was fundamental to belief in him. The three heads were supposed to represent the fact that Triglav ruled over all three major realms of the world: heaven, earth, and underworld. In the source I'm reading, there's an interesting subtext of agency in the three heads, which is to say that the authors come just short of stating that Triglav's appearance is a matter of personal choice. That is, Triglav might have had absolute control over his own appearance, although it doesn't appear that that control was used in any myths if he did. The veils were worn because Triglav took a sort of see no evil attitude concerning the affairs of mortals and thus refused to see anyone's sins.
Also in the town of Stettin were four temples dedicated to the god, the most important of which was decorated extensively. The decorations appear to have consisted of finely embossed figures of people and animals. It was said that none of the decorations ever faded, regardless of weather. Finery was the byword there, and it was customary to offer one tenth of all spoils of war to the temple of Triglav. Consequently, Triglav's main temple was very finely outfitted and contained a large amount of nice stolen stuff. The other three temples were apparently considered to have been less important, and it appears that they were treated much like village churches after the advent of Christianity in that they served as meeting places and public spaces, primarily.
It was very important for the worship of Triglav that a large black horse be available as a sacred animal. No humans were allowed to ride this horse, what with it being sacred and all, and it appears that its main use was in deciding whether or not to declare war. The priests would take the animal in front of the temple of Triglav and lay nine lances about a yard apart from one another. The horse, which would be wearing an empty gold and silver saddle, would then be led over the lances by the head priest. If the lances were undisturbed at the end of the ritual, then it was taken as a favorable omen from Triglav and war was declared.
It is distinctly possible that Triglav was simply a localized version of a much larger cult, that of Svantovit, the king of most Slavic gods. Svantovit, too, had a horse associated with him with which the priests divined the future and was similarly considered to have been all-powerful by his worshippers. Additionally, Triglav is a rather bland name for a god, leading some to believe that it was actually an appellation of some aspect of him that wound up with its own cult. This sort of thing was none too unusual in the fluid religious world that existed before the introduction of Christianity and can be seen in the Romans' veneration of certain aspects of Jupiter (e.g. Iuppiter Stator, etc.). It's not even unheard of in Christianity. Jesus is often referred to as "the rock" or "the savior" or "the son of God," all of which were roles that he played that have been accentuated in art and music such that they have come close to taking on symbolic lives of their own seperate from the way people see the Jesus of the New Testament.
But I digress. Triglav's worship was actually ended by Jesus in his role as smasher of idols. Of the three statues of Triglav that we now know of, only one is said to have survived the regional conversions to Christianity. When Otto, Bishop of Bamberg was tearing around the country destroying all things heathen or pagan, the Slav priests of the town of Wollin took their statue of Triglav and gave it to a woman who lived in the middle of nowhere. She hid it in a hollow tree. A German fellow came by and asked her if he couldn't thank the idol for having saved him from drowning at sea, and she said that that was alright with her. He had been intending to steal the statue, but when he saw it he found himself unable to move it. Whether this was due to the divine influence of Triglav or simply the enormous weight of a golden idol is left to our imaginations, but the German was forced to leave the idol there, making off with only Triglav's gold and silver saddle. The statue at Stettin was broken by the aforementioned bishop Otto, who then mailed one of its heads to the pope. The last statue, at Brandenburg, was broken into pieces by order of Prince Pribyslav, ruler of that country, when he converted to Christianity in 1154.
Source: Gray, Louis Herbert. The Mythology of All Races, Volume III. Boston, 1918.