  This work is devoted to systematically study general $N$-soliton solutions
possibly containing multiple degenerate soliton groups (DSGs), in the context
of the sharp-line Maxwell-Bloch equations with a zero background. A DSG is a
localized coherent nonlinear traveling-wave structure, comprised of inseparable
solitons with identical velocities. Hence, DSGs are generalizations of single
solitons (considered as $1$-DSGs), and form fundamental building blocks of
solutions of many integrable systems. We provide an explicit formula for an
$N$-DSG and its center from an appropriate Riemann-Hilbert problem. With the
help of the Deift-Zhou's nonlinear steepest descent method, we rigorously prove
the localization of DSGs, and calculate the long-time asymptotics for an
arbitrary $N$-soliton solutions. We show that the solution becomes a linear
combination of multiple DSGs with different sizes in the distant past and
future. The asymptotic phase shift for each DSG is obtained in the process as
well. Other generalizations of a single soliton are also carefully discussed,
such as $N$th-order solitons and soliton gases. We prove that every $N$th-order
soliton can be obtained by fusion of eigenvalues of $N$-soliton solutions, with
proper rescalings of norming constants, and every soliton-gas solution can be
considered as limits of $N$-soliton solutions as $N\to+\infty$. Consequently,
certain properties of $N$th-order solitons and soliton gases are obtain as
well. With the approach presented in this work, we show that results can be
readily migrated to other integrable systems, with the same non-self-adjoint
Zakharov-Shabat scattering problem or alike. Results for the focusing nonlinear
Schr\"{o}dinger equation and complex modified Korteweg-De Vries equation are
obtained as explicit examples for demonstrative purposes.
