The Rikti are a secret ancient cult dating back to 2500 BC. The Rikti all shave their heads, and wear large black robes, covering their faces. They believe they can use the ancient power located in ley lines to summon Gil'Gari, their deity. Key members of the Rikti have Truesight, the ability to see magic power. These members can channel small amounts of power from the ley lines throughout Liberty City to use powerful magic spells. They must be careful though; drawing too much power from the ley lines can corrupt the caster, turning them into a monstrosity.
The Rikti can be found throughout Liberty City, but primarily are located in The Heart, where the ley line's power is the strongest.
Truesight allows The Rikti to see both the magic power of ley lines and the expenditure of magic energies from all sources, such as mages or magical items.
Channeling the power of the ley lines can give the Rikti all sorts of powers, from elemental manipulation to summoning servants of Gil'Gari. The closer they are to a ley line, the stronger their abilities. But drawing too much energy at once corrupts them, transforming them into demonic monsters with no willpower. This amplifies their strength tremendously, but they'll never regain their humanity.
The Rikti can also travel through the ley lines, disappearing in an instant and reappearing further along the ley line, anywhere from a few feet to miles.