There was no supernatural element to Sir Charles’s death, but it was staged to look like there was a giant hound that killed him.
Sir Charles Baskerville’s death was described as “sudden and tragic” (ch 2, p. 9). The local paper describes his death as “natural causes” but admits that while there is no reason to suspect foul play, neighbors are suspicious for other reasons.
The circumstances connected with the death of Sir Charles cannot be said to have been entirely cleared up by the inquest, but at least enough has been done to dispose of those rumours to which local superstition has given rise. (ch 2, p. 12)
Charles had problems with his heart, but he was murdered. Stapleton took advantage of his fear of the family curse, the “hound” of the Baskervilles. He acquired a large dog and painted its muzzle with phosphorous so that it would seem to glow. Even Holmes is surprised at the exact looks of the dog.
“There is no smell which might have interfered with his power of scent. .... I was prepared for a hound, but not for such a creature as this.” (ch 14, p. 106)
Since Stapleton knew that the family curse frightened the Baskervilles, he played a trick on Sir Henry and Sir Charles. He had hoped to scare them to death. In the case of Sir Charles, he succeeded. Sir Charles already had a weak heart, and the sight of the supposedly supernatural dog was too much for it.
The gothic elements of Victorian novels involve supernatural elements, but they often have perfectly reasonable explanations. In this case, the supernatural dog did exist, except that while it looked like the scary monster from the legend it was really just a very large dog in disguise.