\documentclass[]{standalone}
\usepackage{adjustbox}
\usepackage{amsmath}
\usepackage{graphicx}
\begin{document}
$\left[
\begin{array}{ccc}
  \ddots & ^{\scriptscriptstyle \text{any Pauli}} &  \\
  & {\rotatebox[origin=c]{-35}{$\scriptscriptstyle XY\text{ diagonal}$}} & \\
  & ^{\scriptscriptstyle I\text{s or }Z\text{s}} & \ddots \\
  \hline
  \ddots & ^{\scriptscriptstyle I\text{s or }Z\text{s}} & \\
  & {\rotatebox[origin=c]{-35}{$\scriptscriptstyle \ \ Z\text{ diagonal}$}} & \\
  & ^{\scriptscriptstyle I\text{s only}} & \ddots
\end{array}
\right]$
\end{document}