  Recently, realizing new fermions, such as type-I and type-II Dirac/Weyl
fermions in condensed matter systems, has attracted considerable attention.
Here we show that the transition state from type-I to type-II Dirac fermions
can be viewed as a type-III Dirac fermion, which exhibits unique
characteristics, including a Dirac-line Fermi surface with nontrivial
topological invariant and critical chiral anomaly effect, distinct from
previously known Dirac semimetals. Most importantly, we discover
Zn$_2$In$_2$S$_5$ is a type-III Dirac semimetal material, characterized with a
pair of Dirac points in the bulk and Fermi arcs on the surface. We further
propose a solid-state realization of the black-hole-horizon analogue in
inhomogeneous Zn$_2$In$_2$S$_5$ to simulate black hole evaporation with high
Hawking temperature. We envision that our findings will stimulate researchers
to study novel physics of type-III Dirac fermions, as well as astronomical
problems in a condensed matter analogue.
