\documentclass[12pt]{article}
\usepackage{amsmath}
\usepackage{amssymb}
\begin{document}
\newcommand{\ra}{\rightarrow}
\newcommand{\T}[1]{\mathbb{T}_{#1}}
\newcommand{\lbe}{<_{\beta\eta}}
\begin{align*}
0 & \lbe 0 \ra 0 && \in \T0 \\
&\left.
\renewcommand\arraystretch{1.3}
\begin{array}{@{}l}
\lbe\displaystyle 0^2 \ra 0 \\
\lbe\displaystyle \cdots \\
\lbe\displaystyle 0^k \ra 0 \\
\lbe\displaystyle \cdots
\end{array}
\right\} && \in \T1 \\
& \lbe 1 \ra 0 \ra 0 && \in \T2 \\
& \lbe 1 \ra 1 \ra 0 \ra 0 && \in \T3 \\
& \lbe 3 \ra 0 \ra 0 && \in \T4 \\
& \lbe 1_2 \ra 0 \ra 0 && \in \T5
\end{align*}
\end{document}
This is basically what the cases environment does except that it has two columns, the brace is on the other side and the \arraystretch is set to 1.2.
Edit:
This is basically how aligned works except that I removed the \null\,.
\documentclass[12pt]{article}
\usepackage{amsmath}
\usepackage{amssymb}
\makeatletter
\newenvironment{alignedleft}[1][c]{%
\RIfM@\else
\nonmatherr@{\begin{\@currenvir}}%
\fi
\if #1t\vtop \else \if#1b \vbox \else \vcenter \fi \fi \bgroup
\def\math@cr@@@{\cr}%
\Let@ \chardef\dspbrk@context\@ne
\default@tag
\spread@equation % no-op if already called
\ialign\bgroup
&\strut@
$\m@th\displaystyle{##}$%
\hfil
\tabskip\minalignsep
\crcr
}{%
\crcr
\egroup
\egroup
}
\makeatother
\begin{document}
\newcommand{\ra}{\rightarrow}
\newcommand{\T}[1]{\mathbb{T}_{#1}}
\newcommand{\lbe}{<_{\beta\eta}}
\begin{align*}
0 & \lbe 0 \ra 0 && \in \T0 \\
&
\def\arraystretch{1.206}
\left.
\begin{alignedleft}
\lbe 0^2 \ra 0 \\
\lbe \cdots \\
\lbe 0^k \ra 0 \\
\lbe \cdots
\end{alignedleft}
\right\} && \in \T1 \\
& \lbe 1 \ra 0 \ra 0 && \in \T2 \\
& \lbe 1 \ra 1 \ra 0 \ra 0 && \in \T3 \\
& \lbe 3 \ra 0 \ra 0 && \in \T4 \\
& \lbe 1_2 \ra 0 \ra 0 && \in \T5
\end{align*}
\end{document}