R'hllor

The Lord of Light (also known as the Red God and R'hllor) is a deity widely worshiped in the continent of Essos. The Lord of Light has been described as a "fire god" and its clergy "fire priests", as worship of the Lord of Light centers around fire and light. It is the majority religion in several of the Free Cities, and extends to Asshai in the distant east. Worship of the Lord of Light is almost unheard of in Westeros.

Beliefs
The religion of the Lord of Light is centered on belief in a single, all-powerful monotheistic god. It espouses a strong dualistic belief, focused on the struggle between the good deity "the Lord of Light" and the evil deity known as the "Great Other". The Lord of Light is the god of fire, which provides light, heat, and life. The Great Other, his eternal enemy, represents darkness, cold, ice, and death.

Essentially a monotheistic religion, followers of Lord of Light consider the "Seven aspects" of the Faith of the Seven to be little more than a thin veneer of justification for worshiping idols. Due to the dualistic beliefs of the religion, its more fanatical priests possess extreme black and white thinking: the Lord of Light is the only true god, and all other gods from other religions - The Old Gods of the Forest, the Faith of the Seven, The Drowned God - are demons and false idols that must be cast down and burned.

The Lord of Light's worship involves the idolization of fire and light. Shadows are important too, as they are created by light. The religion is very focused on prophecy, and on ecstatic visions that are received through communion with the flames. Melisandre claims to receive visions about the future from the Lord of Light by staring into fires.

Unlike the religions of the Old Gods of the Forest in the North or the Drowned God of the Iron Islands, the faith of the Lord of Light is stated to possess several holy texts, similar to the Faith of the Seven.

The followers of the Lord of Light also believe in the coming of a chosen warrior to combat the coming darkness, The Prince That Was Promised, who is prophesied to be reborn "amidst smoke and salt" and wield the sword Lightbringer.

Practices
Most of the worship practices of the Lord of Light involve fire. This can range from simple bonfires, to some extreme cases advocating human sacrifice by immolation. The symbol of the Lord of Light is a fiery heart. The religion is led by the "Red Priests", though both men and women can join the priesthood.

In temples of the Lord of Light, Red Priests conduct bonfires and sing prayers each night, beseeching the Lord of Light to bring back the dawn.

The common prayer of the followers of the Lord of Light uses the line "The night is dark and full of terrors", followed by the response line, "Lord, cast your light upon us."

Season 1
The Red Priest Thoros of Myr was a prominent member of King Robert Baratheon's court before his death.

Season 2
Melisandre, a red priestess of the Lord of Light, has installed herself at the court on Dragonstone and has won the trust of King Stannis Baratheon. Under her influence Stannis has converted to the Lord of Light religion. She believes that Stannis is the chosen servant of the Lord of Light who will destroy his enemies with the flaming sword Lightbringer. She also wins over many converts among Stannis' household, including Matthos Seaworth, though his father Ser Davos remains skeptical. Stannis has also taken the flaming heart, the symbol of the Lord of Light, as part of his personal sigil.

At a ceremony on the beach under the castle, Melisandre burns sacred religious artifacts from the Faith of the Seven, icon statues representing the Seven-faced God, despite Maester Cressen's objections. Later, he tries to poison Melisandre by tainting his own cup of wine and sharing it with her. As he lies dying, Melisandre reveals she is unaffected by the poison due to the protection of the Lord of Light.

Melisandre later seduces Stannis, promising to give him a son. She does become pregnant, but not with a human child. After the failed parley with Renly, she "gives birth" to a monstrous shadow creature in the caves below Renly's camp.

Quaithe is also a follower of the Red God.

In the books
In the Song of Ice and Fire novels, R'hllor is a popular deity in the continent of Essos, worshiped extensively in several of the Free Cities. His symbol is a heart surrounded by fire. The Temple of the Lord of Light in Volantis is larger than the Great Sept of Baelor. There are several different major religions in Essos, but unlike in Westeros few are very widespread, instead each nation or city possessing numerous local religions. However, the Lord of Light religion is probably the most widespread religion in Essos, with worship spanning from the Free Cities in the west to Asshai in the distant east. It isn't the majority religion in every city, i.e. it is a small minority in Braavos, but it is still present there. Nonetheless, it is in a plurality in many regions, and in the southern Free Cities such as Myr, Lys, and Volantis it is certainly the majority religion.

According to George R.R. Martin, the R'hllor religion is strongly influenced by the real-life religion of Zoroastrianism, which in turn influenced Judaism and other real-life monotheistic religions. The central element it borrows is that it is a monotheistic religion with a strong sense of Dualism: there is one true, "Good" God, locked in eternal combat with an evil deity. As part of this dualism R'hllor, who embodies light, fire, and heat, is opposed on the level of primordial forces by the "Great Other" who embodies cold and darkness. Zoroastrianism may have also influenced several later Dualistic belief systems, such as Gnosticism and Manichaeism, which the R'hllor religion also resembles in some respects.

The proper name of the deity is "R'hllor", though he is alternatively called the Lord of Light or Red R'hllor. "R'hllor" might simply be the word for "Lord of Light" in the language of Asshai. The name "R'hllor" has not yet been used in the TV series as of the end of Season 2; for the sake of simplicity, it has consistently used "Lord of Light" so far.