After mal'ganis' death Arthas roamed the lands of Northrend while the powers of Ner'zhul began to corupt and control him.
During the final confrontation with mal'ganis in the first story arc of WC3 mal'ganis makes a comment to arthas implying direct comunication and powers over Frostmourne through the being known as Ner'zhul.
When Arthas Menathal took up Frostmourne it was originally as a gambit for the power he needed to destroy mal'ganis. Weve had enough of the misinformed theroy that frostmourne would corupt you if you equiped it.
