I've only passed through the area a few times and never was able to stop, is this the story you are talking about?
Tonkawa Indians believed ghost fires flickered at the top, and they heard weird creaking and groaning, which geologists now say resulted from the rock's heating by day and contracting in the cool night. A conquistador captured by the Tonkawa described how he escaped by losing himself in the rock area, giving rise to an Indian legend of a "pale man swallowed by a rock and reborn as one of their own." The Indians believed he wove enchantments on the area, but he explained that the rock wove the spells. "When I was swallowed by the rock, I joined the many spirits who enchant this place." The first well-documented explorations of this area did not begin until 1723 when the Spanish intensified their efforts to colonize Texas. During the mid-1700s, the Spaniards made several trips to the north and northwest of San Antonio, establishing a mission and presidio on the San Saba River and carrying out limited mining on Honey Creek near the Llano River.
Looks like a nice place to spend a day. Assuming you can get reservations at the park.