What did Roxas fail to do

*WARNING-SPOILERS*

Towards the end of Kingdom Hearts 358/2 Days, after Xion is defeated by Roxas, she requests that he "stop Xemnas". Roxas unquestionably ventured to The World That Never Was. He was confronted by Riku who defeated him by giving in to the darkness in his heart. Riku gave him to DiZ to have Roxas's memories erased so that Sora can be fully restored.