The ritual stone is a stone located at the Ritual Site(aka the North ) where the Mahjarrat meet every 500 years. It is used in the Ritual of Rejuvenation
A mahjarrat is sacrificed before the stone, and those nearer to the stone will be given more power. For years, the ritual stone's location was unknown until Tale of the Muspah , but was not seen until the Grandmaster quest, The Ritual of the Mahjarrat . In The Tale of the Muspah , the Zarosian Mahjarrat Jhallan hibernates under the stone. According to Jhallan, he is 'directly under' the ritual stone. However, it turns out that the stone was a few steps north of his location. Had Jhallan not been sacrificed, he would have received as much power as Lucien himself.
The Ritual stone is extremely ancient, and was probably brought over by the mahjarrat after they arrived from Freneskae .
Mahjarrat generally have great battles to decide who gets to stand closest to the stone. In the 18th Ritual of Rejuvenation, however, Lucien manipulates the system and declares Jhallan the sacrifice without considering the opinions of others.
The ritual stone possibly shares resemblance to the Evil Creator-god of Freneskae. It is important to note that even if another mahjarrat dies near the stone but was not chosen to be sacrificed(Like Lucien), no energy will be given to the other mahjarrat. Instead, the energy simply dissipates.
The stone seems rather cosmetic than necessary, as the mahjarrat is not sacrificed on it, but infront of it. This raises a question- is the power received dependent on the distance from the sacrificed mahjarrat, or the distance from the stone? This is likely to be an oversight by Jagex.