
\documentclass{article}
\usepackage{fontspec}
\setmainfont{Libertinus Serif}
\pagestyle{empty}
\begin{document}

\Huge η̣ ḍ

\newcommand*{\doteta}{η\llap{ \symbol{"0323}\kern0.1\fontcharwd\font`η}}

\newcommand*{\dotd}{d\llap{ \symbol{"0323}}}

\Huge \doteta{} \dotd

\end{document}
