R'hllor, also known as the Red God and the Lord of Light is a deity worshiped in several of the Free Cities, particularly in Lys, Volantis and Lorath. He is almost unheard of in Westeros. His worship involves the idolization of fire and light. Shadows are important too, as they are created by light.
In the books
In the Song of Ice and Fire novels, R'hllor is a popular deity of the Essos continent, worshiped extensively in several of the Free Cities. His sigil is a heart surrounded by fire.
See also
- R'hllor at A Wiki of Ice and Fire (major spoilers from the books).