Siegel modular forms with respect to non-split symplectic groups

We denote by $G$ the unitary group of the quaternion hermitian
space of rank two over an indefinite quaternion algebra $B$ over
the rational number field. Then the group $G$ is a $Q$-form of $\operatorname{Sp}(2;\mathbb{R})$,
and each $Q$-form of $\operatorname{Sp}(2;\mathbb{R})$ can be obtained in this way.
In this talk, we will consider Siegel modular forms for discrete
subgroups of $\operatorname{Sp}(2;\mathbb{R})$ which are defined from $G$ in the case where
$B$ is division.