Oathkeeper & Oblivion

When Roxas uses his full power, he wields both Oathkeeper and Oblivion. His strength greatly increases when used. The combination of these Keyblades is first used in Kingdom Hearts II when Roxas battles Axel for the second time. It also is used in the last mission of Kingdom Hearts 358/2 Days, before Roxas's battle with Riku. In the Mission Mode of Kingdom Hearts 358/2 Days, equipping the Zero Gear with 3 ability units attached will result in a hooded Roxas wielding Oathkeeper and Oblivion. His Limit Break will change to Magic Hour, a powerful Limit Break in which he calls down grand beams of Light. Roxas gained the ability to use both of these weapons together at once after the death of Xion.

Speculation
When Roxas was first seen wielding these dual Keyblades in the secret movie in Kingdom Hearts, he was only known as BHK (Blonde-Haired Kid). An important quote led to much speculation about who Roxas was made by Nomura in an interview before the release of Kingdom Hearts II:

"The Keyblades the boy who knows Sora is holding are the Oathkeeper and Oblivion from the first game. The precious "Oathkeeper", which Sora received from Kairi. The jet-black Keyblade, symbolic of Riku. / The Oblivion, at first, was called Riku's Keyblade. The boy who has the Keyblades from Riku (darkness) and Kairi (light)...... Who is he!?" - Nomura

Trivia

 * You can't Dual-Wield Oathkeeper and Oblivion in Kingdom Hearts 358/2 Days's Story Mode. Equipping the Zero Gear with 3 ability units in Story Mode will result in the Kingdom Key+ with all of Oathkeeper/Oblivion's abilities.
 * When Dual-Wielding Oathkeeper and Oblivion in Mission Mode, Roxas is very weak compared to his strength during the last mission in the game's Story Mode, before the battle with Riku, where he can destroy Neoshadows rather easily. However, it should be noted that the enemies in Mission Mode have buffered stats for more players.
 * Roxas's ground combo with the Oathkeeper and Oblivion in Kingdom Hearts 358/2 Days is exactly the same one from his battle in Kingdom Hearts II Final Mix.
 * Curiosly enough, when fighting against Xion in the Mission mode, Dual Roxas makes her more damage than other characters, even Xion herself. This could be because you obtain the Oathkeeper/Oblvion keyblades by absorbing Xion,and thus giving Roxas more power to defeat her. This makes Dual Roxas a good choice for battle against Xion.