After the destruction of Marienburg, Carroburg and Altdorf, Nuln became the greatest city of the Empire. However, once again it was threatened by Skaven, who sought to overrun it through the sewers. The 4th end times book reveals that Nuln was actually the first of those cities to be destroyed, overrun by the skaven. hmm. Use of grenades to combat them led to massive upgrades to the sewer system including Gromril grates, leading to greater levels of hygiene and overall growth of the city and a new holiday past time for the denizens of Nulndon...
The Nulndon meme originated in an October 2014 Warhammer Fantasy General thread on /tg/. Originally instigated by a conversation beginning in an evaluation of the industries remaining in the Empire after the attacks of Nurgle's forces during The End Times event, a series of jokes reminded the fa/tg/uys why Nuln is the only Empire city most people remember.
The thread chain begins here: 
"Hey, Hans, the sewers are talking again. Mind tossing a few grenades down there?"
Hans and his keen eared companion are the unlikely heroes of Nuln and their timely inconvenience absolutely squashed several incursions of Skaven attacks. This method of handling Skaven dweller attacks spurred a new holiday in Nulndon, celebrating the successes and triumphs of the city by summarily tossing grenades down the sewers. The Grenade and Sewer Gate becomes the symbol of the Guild of Urban Hygiene and Sanitation.
Several anons have postulated just who Hanz is and why he is so uncannily everywhere exactly where he needs to be. Some think he's a low born human hero who just chances his way through the ranks of the empire and remains humble. Some think that he's quite possibly the luckiest man alive and just happens to be everywhere where the action and fighting gets roughest. Lastly some think that just maybe he's a Tom Bombadil (emphasis on bomb) and passes off as a human as to not draw attention to his infinitesimal power levels and just trolls evil wherever it may be with grenades as a show of just how easily he can dispatch the horrors of the Old World. Whatever the case may be, the mystery of Hanz is inspiring. It matters not who or what he is, the myth is what is most important, and the simple charm and reliable timeliness of Hanz's presence wherever danger is most present is what makes him so lovable.
We know one thing. Hanz is one bitch of an inconvenience to the enemies of the Empire, wherever they may be.
The Fall of Nuln
However the hopes of /tg/, it's been stated in the White dwarf End Times' cronology () that Nuln falls en the early half of 2525, by a skaven force, led by Thanquol and Skreech Verminking, one of the Verminlords. At the end of End Times regardless, all of the Warhammer setting is destroyed.