Illyrio Mopatis

Illyrio Mopatis is a character in Game of Thrones. In the unaired pilot episode he was played Ian McNeice, but for the series itself was replaced by Roger Allam due to a scheduling conflict.

Illyrio Mopatis is a Magister or merchant-lord of the Free City of Pentos, located across the Narrow Sea to the east of Westeros. Immensely rich and powerful, Illyrio is a friend and ally to Viserys and Daenerys Targaryen.

Season 1
Winter is Coming Illyrio brokers the deal to 'sell' Daenerys in marriage to Khal Drogo of the Dothraki. Drogo desires an exotic wife from far away and Viserys needs a Dothraki army to help retake the Seven Kingdoms, so a bargain is made. Illyrio hosts the wedding and gifts Daenerys with three dragon eggs, turned to stone by the passing of ages.

The Wolf and the Lion
Illyrio pays a secret visit to King's Landing where he discusses their mutual plans with his ally, Varys. Once Khal Drogo's son is born, the Dothraki can move to invade Westeros and Viserys can retake the Iron Throne. However, Westeros must be in chaos and civil war before this happens. When Varys warns that a civil war, launched by a conflict between House Stark and House Lannister, is imminent, Illyrio becomes concerned that things are moving too fast, but Varys suggests motivating the Dothraki to move faster might be a viable strategy. Their conversation is overheard by Arya Stark, but she doesn't really understand it and is only able to relay a few garbled words to her father.

In the books
In the Song of Ice and Firenovels, Illyrio is a fat, middle-aged man who dresses in rich finery. He favors using money and wealth to overcome problems rather than honor and valor. He is politically canny and astute.