The Dark Lady and the Bright Lord

Among the Dragonborn, it is slightly unlucky to name the twin Gods that they venerate, except in prayer and worship. Outside of this formal environment, they call them the Dark Lady and the Bright Lord.
The Bright Lord is Bahamut, the Platinum Dragon, God of Chivalry, Honor and Law.
The Dark Lady is Tiamat, the Chromatic Dragon, Goddess of Treachery, Intrigue, and Power.
Both of these deities were created when the progenitor Dragon god Io was slain by a Primordial during the Dawn War. According to legend, the Primordial split Io into two halves. These two halves became Bahamut and Tiamat.
It is interesting to note that most Dragonborn Paladins will swear by the Bright Lord, while most Dragonborn Warlocks will choose an infernal pact and swear by the Dark Lady.