During the Narnian Revolution, Sopespian plotted with Lord Glozelle to overthrow their king Miraz and take over the kingdom. They manipulated Miraz into accepting the challenge to a duel with Peter Pevensie, in which they hoped he would be killed.
Sopespian and Glozelle were Marshals of the Lists at the duel. In the middle of the duel, the two lords accused Peter of foul play, led the Telmarine army in a charge, and used the chaos to kill Miraz.
The Telmarine Army, led by Sopespian, rushed at the Old Narnians, commencing the Second Battle of Beruna. Immediately, Peter Pevensie slashed at Sopespian's legs with his sword, and with the back-cut of the same stroke decapitated him.
Sopespian and Glozelle were both tall men who wore capes and pointed caps.
In the Disney film version, Sopespian was the one who killed Miraz, stabbing him with an arrow and saying that the Narnians shot him, and Gregoire is the lord killed by Peter.
When the Trees give the Narnians the upper hand, Sopespian tells the Telmarine Army to retreat to the Bridge of Beruna (which his men constructed earlier to get the troops to Aslan's How), but when they got there, they found Lucy and Aslan waiting for them.
Aslan roared and awoke the River God, which destroyed the bridge and, literally, swallowed Sopespian whole, consuming him in his waters, no doubt crushing him to death.
In the BBC TV show, his role is the same as in the book, except it is Glozelle who is killed by Peter, and Sopespian's fate remains unknown. However, he is not seen at the end, implying he was killed in the battle.