  In connection with our previous investigation about Siegel threefolds which
admit a Calabi--Yau model, we consider ball quotients which belong to the
unitary group $\U(1,3)$.
  In this paper we determine a very particular example of a Picard modular
variety of general type. Really we determine the ring of modular forms. This
algebra has 25 generators, 15 modular forms $B_i$ of weight one and ten modular
forms $C_j$ of weight 2. Both will appear as Borcherds products. We determine
the ideal of relations. The forms $C_i$ are cuspidal. Their squares define
holomorphic differential forms on the non-singular models.
