  We discover a large class of simple affine vertex algebras $V_{k} (\mathfrak
g)$, associated to basic Lie superalgebras $\mathfrak g$ at non-admissible
collapsing levels $k$, having exactly one irreducible $\mathfrak g$-locally
finite module in the category ${\mathcal O}$. In the case when $\mathfrak g$ is
a Lie algebra, we prove a complete reducibility result for $V_k(\mathfrak
g)$-modules at an arbitrary collapsing level. We also determine the generators
of the maximal ideal in the universal affine vertex algebra $V^k (\mathfrak g)$
at certain negative integer levels. Considering some conformal embeddings in
the simple affine vertex algebras $V_{-1/2} (C_n)$ and $V_{-4}(E_7)$, we
surprisingly obtain the realization of non-simple affine vertex algebras of
types $B$ and $D$ having exactly one non-trivial ideal.
