An old oak struck by lightning burning from the inside. For Slavs, trees struck by lightning were believed to ward off demonic forces and possess purifying power. Probably because "Perun, the thunder god, now resided in them"?
Serbian hollow oak chapels
This particular one is dedicated to St Pantelija which was built inside of a hollow ancient oak near the village of Jovac, Vladičin Han region, South-eastern Serbia. The chapel was built by a local villager Dragoljub Krstić in 1991.
St Pantelija to whom the chapel is dedicated is the last of the "fiery" summer saints. He is celebrated at the beginning (9th) of August at the end of Kresovi, the fire days, the days of the highest heat. St Pantelija is in the Serbian folk tradition the brother of St Ilija the Thunderer (Sveti Ilija Gromovnik). St Ilija the Thunderer is in the Serbian folk Tradition the saint who controls the thunder. His day is the 2nd of August which was originally the day of Perun. This means that both St. Ilija the Thunderer and St. Pantelija Christian personifications of Perun, the thunder god, the third and most powerful head of Triglav.
It seems fitting that the chapel dedicated to the St Pantelija is inside of an Oak, the holy tree of the thunder gods.