Geralt meets him first, when he is talking to King Foltest, in the Camp of Foltest's army during the Prologue.
Geralt meets him again in Act 2 in Henselt's military camp. The witcher learns the ambasador kindapped Triss trapped in a statue form and sailed to Loc Muine. There he orders Nilfgaardian sorceress Assire var Anahid to release Triss from the statue form and orders to kill Assire for conspiracy against emperor Emhyr var Emreis. Later he tortures Triss to learn more about Lodge of Sorceresses.
In Act 3 if Geralt chooses to rescue Triss from the Nilfgaardians, he takes Shilard hostage. When they come before a company of soldiers, their leader trains a crossbow on Shilard, revealing that he was ordered by the capital to execute him and kills Shilard.
If Geralt doesn't rescue Triss, Shilard uses Letho to frame the Lodge of Sorceresses for the regicides.
The Witcher 2: Assassin of Kings
The Witcher 3: Wild Hunt