  Employing inelastic X-ray scattering and neutron scattering techniques, we
observed nematic and magnetic phase transitions with distinct characters in
K$_{5}$Fe$_{4}$Ag$_{6}$Te$_{10}$. Upon cooling, the nematic order undergoes a
strongly first-order phase transition followed by a second-order magnetic
transition at $T_{\textrm{N}}$ $\approx$ 34.6 K. The temperature difference
between these two phase transitions is $\sim$ 1 K. The observed phenomenon can
be attributed to a distinctive first-order preemptive Ising-nematic transition,
a characteristic unique to a quasi-two-dimensional scenario marked by strong
out-of-plane spatial anisotropy due to weak coupling. Our studies establish
K$_{5}$Fe$_{4}$Ag$_{6}$Te$_{10}$ as the first material in the family of iron
pnictides and chalcogenides that possesses a nematic tricritical point
preceding the magnetic one upon decreasing nematic coupling.
