The Walhalla is a 
Bavarian decision, which builds the Memorial Walhalla in Regensburg.
The requirements are that Bavaria have researched Romanticism and controls the province of Regensburg (which it does from the start of the game). It is also possible to take if Bavaria forms 
South German Federation or 
Germany.
It will give +0.01 prestige pr month for the rest of the game (adding up to 12 pr. decade). Thus it is better to take the decision as early as possible to get the biggest amount of prestige possible out of it.
Ingame description
Constructed between 1830 and 1842, the Walhalla is a hall of fame in Regensburg, Bavaria, housed in a neo-classical temple building, overlooking the Danube. The brain child of King Ludwig I, the hall commemorates famous personalities from German history from Arminius to Copernicus.;Construit entre 1830 et 1842, le Walhalla est dédié aux grands hommes. Situé à Regensburg, en Bavière, il est abrité dans un temple néoclassique surplombant le Danube. Voulu par le roi Ludwig Ier, ce bâtiment rend hommage aux personnalités de l’histoire de l’Allemagne telles qu’Arminius ou Copernic.;Die zwischen 1830 und 1842 in Regensburg, Bayern, gebaute Walhalla ist eine Ruhmeshalle, ein klassizistisches, die Donau überschauendes Tempelgebäude. Die Halle, erdacht von König Ludwig I., erinnert an berühmte Persönlichkeiten der deutschen Geschichte, von Arminius bis Kopernikus.