As long long ago, Thrall told Shalaman to regain his honor in the horde, He would have to gather an army and face the horrors of Naxxramas since Thrall's mightiest warriors had not even gained one source of information to be able to defeat Kel'Thuzad or his minions.
He went to dalaran to ask Rhonin for help. Rhonin couldnt come himself but he sent his best magicans and warriors to break the defenses of Naxxramas.
They went to naxxramas and began their assult.