Enter the tavern and speak with Ser Gilbert Farring.
Since it's one of the only two-handed blades that can be used on a horse, it makes a good substitute for a lance if you have more points in two-handers than polearms. It makes a good weapon for duels against armored opponents but if you get ganged up on it's better to switch to another weapon. It is essentially a large rapier, and is a good piercing weapon but it's not as great for slashing. The Braavosi Great Sword is a unique two-handed blade, won as a quest reward from the quest 'Seed of the Dragons'.