Penetrating hurts
Anub'arak was sent south to meet Arthas Menethil when he arrived in Northrend. Anub'arak saved the dark prince from an attack by blood elves, before he introduced himself and told Arthas of Ner'zhul's plans. Anub'arak helped Arthas to establish a base, and then masterminded the attack on the blue dragon Sapphiron. Anub'arak told Arthas that it would take far too long to fight their way through Kael'thas and Vashj's expansive forces, and formulated a plan to go through the underground ruins of Azjol-Nerub to reach Icecrown, but warned that it would be a perilous journey.
Anub'arak led Arthas into the ruined Spider Kingdom, negating many perilous traps along the way, but all the while, he fought many of his former subjects, the living nerubians, who called him the traitor king for serving the Lich King, who had destroyed Azjol-Nerub.
As they entered the kingdom, they encountered survivors of Muradin's expedition, now led by Baelgun, who reported that an ancient evil had been released beneath the kingdom, and that they would not allow any to open the Inner Kingdom's doors. Anub'arak and Arthas defeated the troublesome dwarves and broke into the Inner Kingdom. Soon, they came upon the evil the dwarves had spoken of - the Faceless Ones - strange, malformed creatures who had been imprisoned beneath the earth in ages long forgotten. They even faced off with a Forgotten One, a creature of great power and resilience.
Anub'arak in Warcraft III: The Frozen Throne.Upon entering the Upper Kingdom, an earthquake separated Anub'arak from Arthas, and Anub'arak and his minions dug fervently through the rubble to get to Arthas before he killed himself in one of the many booby traps laden throughout the halls of Azjol-Nerub's Inner Kingdom. Impressed at Arthas' resilience, when he found him, Anub'arak quickly ushered him out of Azjol-Nerub and to the surface, where they were immediately engaged with Illidan Stormrage and his servants in a battle for the Frozen Throne.
Anub'arak was quick to the battle, defending Arthas as he magically activated the four obelisks surrounding Icecrown's pinnacle. Finally, all four were active, and Anub'arak stood by as Arthas climbed the stairs to his destiny.