This is the famous Hindsgavl Dagger.
The following is an excerpt of the FlintForum digest, from Mike Stafford who is the curator of Anthropology, Cranbrook Institute of Science in Michigan:

"The dagger was found at Hindsgavl, Denmark, a town on the south-central island of Fyn. To my eye it is the most impressive stone tool made anywhere, anytime. But I'm also biased towards the Danish stuff. This dagger is actually close to 12" long and is unbelievably wide. The blade is finished in massive pressure flakes over a ground surface. The reddish color is from bog patination over translucent Danish flint. Chronologically this dagger dates to near 1900 BC. It is technically classified as a type IVe in the dagger typology established by Lomborg. This dagger, considered a Danish national treasure, is on permanent display in the Danish National Museum in Copenhagen".