Although Ner'zhul obtained unparalleled mental power with the help of the Helm of Domination, allowing him to control the massive undead army and spread his will out, he couldn't move himself. Kil'jaeden had imbued powerful magic on the ten-thousand-year-old piece of ice he collected from Northrend. Although this piece of ice was called the Frozen Throne, it was actually Ner'zhul's cage.
Therefore, after calculating in his heart for a long time, Ner'zhul found Tichondrius and Mal'ganis.
Ner'zhul knew very well these dreadlords were the prison guards that Kil'jaeden had sent, but he had no choice. Most of his subordinates were brainless undead, so he could only find the dreadlords to cooperate with his plan.