Arthas’s reference is the Scourge from the Warcraft fictional universe

Warcraft is a franchise of video games, novels, and other media created by Blizzard Entertainment. Warcraft is one of the highest-grossing video game franchises of all time, having grossed $11.2 billion in lifetime revenue, as of 2018. All games in the series have been set in and around the world of Azeroth, a high fantasy setting. Initially, the start of the series focused on the human nations that make up the Eastern Kingdoms, and the Orcish Horde, which arrived in Azeroth via a dark portal, beginning the great wars. Later on, in the series, the world of Azeroth was expanded, revealing the new continents of Kalimdor, Northrend, Pandaria, Broken Isles, Kul Tiras, and Zandalar, allowing the introduction of the Night Elves, Tauren, Pandaren, and other major races into the universe.

The Scourge (sometimes called the Undead Scourge or Scourge Army) is one of three major undead factions existing in the world of Azeroth. Until recently, it was a major player in control of the world. Created initially as the precursor to an invasion by the Burning Legion, the Scourge broke free from their demonic masters and under the iron rule of the dread Lich King built up their power base on the arctic continent of Northrend.

Their influence spreads throughout Northrend, as well as the Plaguelands in northern Lordaeron and southern Quel'Thalas, and even Kalimdor to a small degree. Consisting of thousands of walking corpses, disembodied spirits, beasts of the north, and damned mortal men, the Scourge is a terrifying and insidious enemy which, before its downfall, was perhaps the most dangerous threat to the world of Azeroth.

The Scourge was (and still is) a vast army of the undead serving the Lich King – the being formerly known as the orc shaman Ner'zhul. Under the control of the Burning Legion, the Scourge's mission was to spread terror and destruction across the world in anticipation of the Legion's inevitable invasion. A mission that was almost undone before it even began when the nerubians attacked and began the War of the Spider, however, the conflict ultimately ended with the Lich King's first major triumph (while the nerubians were immune to the plague, their corpses could still be reanimated). The Lich King was impressed by his enemy, however, and adopted nerubian architecture for his own, as a testament to the spider lords' tenacity and age.