>>715127441So, Ner'zhul was completely and totally without influence or power inside of the block of ice. He used what little bit of power he had with a non-physical body to push Forstmourne out of the ice, which was basically jettisoning a thing into the mortal world where it COULD be found and used.
Upon Arthas' arrival in Northrend, his very nature made him susceptible to the 'call' of Frostmourne, which, the intent of Ner'zhul was to get the fuck out of the ice and escape the yoke of the Burning Legion. Arthas finds it, is corrupted fully and absorbed into the Sword, breaks Ner'zhul out and puts on the helm. Arthas' spirit is more powerful than Ner'zhuls, so after a brain battle inside of the Helm of Domination, Arthas emerges the controlling power owner. At this point, everything that happens is the desire of Arthas (admittedly, a corrupted and evil Arthas) and not solely Ner'zhul.
TL;DR, Ner'zhul wanted a bitch to get him out and Arthas was like 'under new management lmao'