This diploma thesis deals with the preparation and characterization of high-entropic ceramics based on the garnet structure, specifically (Er0.05Yb0.35Y0.2Lu0.2Gd0.2)3Al5O12, and studying its optical properties. The powder precursor was prepared using combustion synthesis, which made it possible to achieve atomic homogeneity already at the liquid phase stage. The rapid combustion process prevented unwanted separation of components, which turned out to be crucial for the subsequent formation of a stable high-entropic phase. The prepared spherical nanoparticles provided a high driving force for the sintering process of the material. The compaction process itself was carried out using advanced Spark Plasma Sintering (SPS) technology. The critical sintering temperature was determined to be 1450 °C, at which a relative density of 99.9% was achieved. Research into the influence of the applied mechanical pressure on the final microstructure and density during SPS showed that a value of 30 MPa was sufficient for the given system and further increase up to 70 MPa did not lead to a significant improvement. In terms of heating rate, it has been shown that it does not have a significant effect on the resulting average grain size, but it significantly affects the optical properties. At low heating rates, it promoted undesirable surface diffusion in the medium temperature range, which led to premature closure of pores at grain boundaries without effective densification of the material. Rapid heating (100 °C/min) eliminated these negative factors and suppressed surface diffusion and allowed a rapid transition to the high temperature range, where dominant volume diffusion ensured effective displacement of residual porosity, which was crucial for achieving translucency of high-entropic YAG. Structural analysis confirmed that the resulting material was composed of five evenly distributed rare earth elements (Er, Yb, Gd, Lu, Y), which occupy dodecahedral positions of the garnet lattice. Photoluminescence measurements at 980 nm excitation demonstrated successful activation of Er3+ and Yb3+ ions, with up-conversion spectra showing characteristic green and dominant red emission. The final product is a translucent ceramic with high phase purity, which represents a stable platform for advanced optical and luminescent applications.