Secret marriage pact
The secret marriage pact was a clandestine agreement signed in Braavos many years ago forging an alliance between Dorne and primarily Viserys Targaryen. Its purpose was to aid in the overthrow of King Robert Baratheon and restore House Targaryen to the Iron Throne. The alliance was to be sealed by a marriage between Viserys Targaryen and Princess Arianne Martell. Viserys died unaware of the secret pact. Dany was not made aware of the pact until she became Queen of Meereen.
The pact was made on parchment and was written in the Common Tongue. It has seals, with gold and ribbons and signatures. It is yellowed with age. It was made in Braavos when Daenerys was a little girl. Ser Willem Darry signed on behalf of (presumably) House Targaryen. Prince Oberyn Martell signed for Dorne, with the Sealord of Braavos as witness. The pact verbatim has not been revealed (as in, for example, the Bastard Letter).
According to Ser Barristan Selmy the secret marriage pact contains “not a word” about Daenerys Targaryen although Dany does say after reading the pact that Ser Willem Darry signed “for us”, she dies not elaborate on exactly who “us” are, it may be House Targaryen in general or she may have meant her brother. It is not known if Varys knows of the pact.
A Feast For Crows
Prince Doran Martell finally reveals to Arianne Martell that she had secretly been promised to the now-dead Viserys Targaryen in an effort to undermine those who lead to Elia's death. He tells her that the man she was promised to is dead, putting an end to careful planning. He tells her than Dorne will be hers.
He tells her that her brother has a harder road to walk than she. Doran’s new plan is for Prince Quentyn travel across the Narrow Sea to meet Viserys's sister Daenerys and wed her instead.
A Dance with Dragons
Prince Quentyn Martell journeys to Meereen with the pact parchment hidden in a flap in his boot. Once presented to Daenerys Quentyn and his two companions reveal themselves to the Queen as Westerosi knights. Quentyn hands the parchment, which he refers to as a gift, to Daenerys so that she can read it. When Ser Barristan Selmy asks her what the parchment says Dany tells him and hands it to him saying,
As Ser Barristan Selmy reads the pact her remarks to Dany,
Dany replies that no doubt that was why Prince Doran chose to keep the pact a secret, as
“If my brother had known that he had a Dornish princess waiting for him, he would have crossed to Sunspear as soon as he was old enough to wed.”
Prince Quentyn then reveals his true identity to Dany. Dany immediately surmises that with Viserys dead he means to marry her, that it is now she who must seal the pact if she wants Dorne. Quentyn tries to convince Daenerys to marry him, telling her that it will mean 50,000 Dornish swords pledged to her service. The Queen, however, does not accept Quentyn's proposition. She tells him of her upcoming marriage to Hizdahr zo Loraq, and remarks,
“Would that you had come a year ago.”
Quentyn tells her that it's not too late but Dany replies that she will be the judge of that. After she dismisses her audience she examines the pact again. She realizes that it was executed in Braavos, while they were living in the house with the red door. She wonders why that makes her feel strange. Daenerys soon goes through with her marriage to Hizdahr zo Loraq. Although Daenerys is courteous to Quentyn and his companions, she urges Quentyn to go home. 
After hosting the wedding feast, Daenerys is counselled by Ser Barristan. Their talk turns to her options; he reminds her that there is another road, Dany knows he means the Dornish Road. He tells her that House Martell is ancient and noble and has been a leal friend to House Targaryen for more than a century. Dany replies,
“It would please me if he [Quentyn] had turned up with those fifty thousand swords he speaks of. Instead he brings two knights and a parchment. Will a parchment shield my people from the Yunkai’i? If he had come with a fleet …”
Daenerys has already decided against the Dornish alternative in her own mind, she tells Ser Barristan,
”Dorne is too far away. To please this prince, I would need to abandon all my people. You should send him home.”
After Hizdahr zo Loraq assumes control of Meereen in Daenerys’s absence Ser Barristan realises that Quentyn’s continued presence is a provocation for Hizdahr.  In the audience chamber of the Great Pyramid Ser Barristan warns him to leave the city of Meereen.  When Quentyn tries to mention marriage pact, Ser Barristan immediately interrupts him, saying,
“[the marriage pact] - was made by two dead men and contained not a word about the queen or you. It promised your sister’s hand to the queen’s brother, another dead man. It has no force. Until you turned up here, Her Grace was ignorant of its existence, your father keeps his secrets well, Prince Quentyn. Too well, I fear. If the queen had known of this pact in Qarth, she might never have turned aside for Slaver's Bay, but you came too late, I have no wish to salt your wounds, but Her Grace has a new husband and an old paramour, and seems to prefer both of them to you.”
“You were promised, Arianne.”
– Prince Doran, to his daughter
“Your father keeps his secrets well, Prince Quentyn. Too well, I fear."
- Ser Barristan Selmy
“It has no force."
- Ser Barristan Selmy