Infernal Power is an emote unlockable by members using Loyalty points. It is bought from Xuan in either Burthorpe or Varrock for 12,000 loyalty points. It was released on the 6th of December, in the third batch of loyalty rewards update.
When seen by the character point of view, it seems like there is an inverted 5-pointed star (or pentagram) surrounded by a circle is drawn on the ground. This is a reference to the Sigil of Baphomet which is the official insignium of the Church of Satan, thus it makes sense, as it is the Infernal Power.