Following the recommendation of Steven B. Segletes, I decided to compile a standalone Tengwar text. It is possible to specify a transparent background with \nopagecolor using the xcolor package.
\documentclass[convert={convertexe={convert}}]{standalone}
\usepackage[all]{tengwarscript}
\usepackage{xcolor}
\begin{document}
\color{white}
\nopagecolor
\tengwarannataritalic
\tengwa{254}
\Textendedcalma\TTthreedots\Tnuumen\Tessenuquerna\TTthreedots\Tungwe\Tando\Toore\TTrightcurl\Tumbar\Ttinco\TTthreedots\Tlambealt\TTrightcurl\Tquesse\TTdoublerightcurl
\Tromanperiod\Ts
\Textendedcalma\TTthreedots\Tnuumen\Tessenuquerna\TTthreedots\Tungwe\Tungwe\Tumbar\TTnasalizer\TTdot\Ttinco\TTthreedots\Tlambe\TTrightcurl
\tengwa{255}\\
\Textendedcalma\TTthreedots\Tnuumen\Tessenuquerna\TTthreedots\Tungwe\Tthuule\Troomen\Tquesse\TTthreedots\Ttinco\TTthreedots\Tlambealt\TTrightcurl\Tquesse\TTdoublerightcurl
\Tromanperiod\Ts
\Textendedungwe\TTthreedots\Tumbar\Toore\TTrightcurl\Tesse\Tkern{-0.2}\Tmalta\TTrightcurl\Textendedcalma\TTdot\Ttelco\TTdot\Tquesse\Troomen\Tparma\TTnasalizer\TTdot\Ttinco\TTthreedots\Tlambe\TTrightcurl\\
\end{document}

To get the right colors for a Beamer presentation using the orchid theme, I used this guide, and apply them to the text. Then, in the Beamer, I just \includegraphics the output Tengwar text PDF (named one_ring.pdf):
\documentclass{beamer}
\usecolortheme{orchid}
\begin{document}
\begin{frame}
\begin{block}{\includegraphics[scale=0.55]{one_ring}}
This inscription says in Black Speech:
\textit{Ash nazg durbatulûk, ash nazg gimbatul,}\\
\textit{Ash nazg thrakatulûk agh burzum-ishi krimpatul.} \\~\\
Translated, the words mean:
\textit{One ring to rule them all, one ring to find them,}\\
\textit{One ring to bring them all and in the darkness bind them.}
\end{block}
\end{frame}
\end{document}

standalonedocuments and include elsewhere as graphics. – Steven B. Segletes Nov 19 '19 at 13:40