He is an immensely wealthy individual who has grown rich from the training and trading of the Unsullied warrior-eunuchs.
Kraznys mo Nakloz is one of the Good Masters, the ruling elite of the city of Astapor, composed of slave traders. As with many of his compatriots, Kraznys only speaks a Low Valyrian dialect and does not know the Common Tongue of Westeros. Thus when dealing with foreign customers, Kraznys relies on his slave Missandei to serve as his translator.
Kraznys, with Missandei at his side, meets with Daenerys Targaryen and shows her some of his Unsullied. These Unsullied had been standing for an entire day without being given food or water. Through Missandei Kraznys explains the Unsullied are trained to fight from the age of five and that only one in four recruits survives the training. To demonstrate that the Unsullied do not fear pain or death, he slices off the nipple of one soldier in the presence of his prospective customers. He then adds that the Unsullied are also trained to show no mercy or weakness by killing a newborn slave child in front of its mother at the end of their training. Kraznys tells her that eight thousand soldiers are for sale and that she has one day until the next day to make a decision. Throughout the meeting, Kraznys insults Daenerys, believing that she doesn't understand Valyrian. Missandei translates Kraznys insult-laden sentences into more respectable phrases.
The following day, Daenerys meets with Kraznys and two other Good Masters, Greizhen mo Ullhor being one of the other two. Kraznys is initially dismissive of her offer to buy all the eight thousand Unsullied and all the boys in training her offer, claiming that she can afford barely more than one hundred Unsullied. In response, Daenerys states she has dragons. After some haggling, Kraznys reluctantly agrees to sell her all of his Unsullied in return for Drogon. In addition, Daenerys demands to have Missandei as a token. Eager to possess a dragon of his own, Kraznys agrees.
Kraznys and his fellow Good Masters are present at one of Astapor's plazas to consummate the transaction and see the dragon for themselves. All the Unsullied soldiers are also assembled. Daenerys and her entourage arrive with Drogon in a cage. Kraznys tells Daenerys that the Unsullied had not seen combat before and advises her to attack some minor cities and towns before proceeding to Westeros. He offers to purchase any healthy slaves that she may capture and some of these would even be trained as Unsullied. Daenerys hands Drogon, who is wearing a long chain leash to Kraznys, who gives her a scourge with a harpy-shaped hilt, signifying ownership of the Unsullied. This symbolic gesture concludes the transaction.
While Kraznys struggles to control Drogon, Daenerys addresses the Unsullied in Valyrian and tests them with battle commands. Krazyns then complains to Daenerys that the dragon is not obeying his commands. In response, Daenerys retorts in perfect Valyrian that "a dragon is not a slave". Kraznys is surprised that Daenerys speaks Valyrian, Daenerys declares that she is Daenerys Stormborn of House Targaryen, of the blood of Old Valyria: Valyrian is her mother-tongue, and thus she understood every word he said to her.
Daenerys orders the Unsullied to kill all of the Good Masters in Astapor, along with all of their household soldiers and any man who holds a whip, to spare only the children and to liberate all slaves. As the Unsullied turn on the slave-drivers, Kraznys frantically shouts for someone to kill Daenerys but the Unsullied, no longer his, ignore him. Daenerys turns back to Krazys and calmly speaks one word: "dracarys". Drogon then breathes fire onto Kraznys, completely engulfing the slaver in flames. Following the Sack of Astapor, Kraznys' charred remains are left unburied.
A master slaver in Astapor, Kraznys trained and traded the city's famous Unsullied soldiers. He agreed to sell his entire army to Daenerys in exchange for a dragon, but Drogon killed him at Daenerys' command.