The Niederwalddenkmal is a minor decision for 
Germany. It is part of the chain of decisions about the Rhine crisis together with The Left Bank and Die Wacht am Rhein
After the Prussian victory and eventual formation of the German Empire, a monument was built to celebrate the unification. Wilhelm I commissioned Karl Weisbach, the architect, and Johannes Schilling, the sculptor, to build the Niederwalddekmal. The work was estimated at the value of one million gold marks. There's an inscription that reads In memory of the unanimous victorious uprising of the German People and of the institution of the German Empire 1870-1871.
You need to have the decision Die Wacht am Rhein enacted, and not be at war with anyone, to make it appear. Once the decision has appeared, Germany needs to be a Great Power and have Nationalism and Imperialism researched.
All of your population will lose 1 militancy and furthermore the decision removes 3 infamy. It is best to use it when your population becomes rebellious because of all the wars Germany participated in.
Ingame description
The Niederwalddenkmal is a monument in the Niederwald park, Hesse, Germany. The monument's central figure is a ten meter high statue of Germania, the personification of the German nation, holding aloft the recovered Imperial crown of Germany, flanked by two statues representing War and Peace respectively. Built between 1871 and 1883, the monument was commissioned after the Franco-Prussian war, to commemorate the founding of the German Empire. Let us all rejoice in the glory of our nation united by erecting a monument to stand the tests of time!