Solid-state reaction at 1000 °C produces a series of Li-stuffed Li5+2x(La1-yEuy)3(Ta1-xZrx)2O12 garnet phosphors (x = 0-1, y = 0.05-0.6) that exhibit favorable efficiency and thermal stability for red luminescence under either blue or n-UV light excitation, where the optimal composition was identified to be x = 0.5 and y = 0.4. The concentration quenching of luminescence was determined to occur via electric dipole-dipole interactions. Zr4+ substitution for Ta5+, accompanied by additional Li+ for charge compensation, was found via Rietveld structure refinement and Raman/UV-Vis spectroscopy to profoundly affect the tetrahedral and octahedral occupancies of Li, the symmetry of (La/Eu)O8 dodecahedron, and the bandgap of the host lattice and cation disorder, with which the systematically varying excitation and emission behaviors of Eu3+ were deciphered. The Li6(La0.6Eu0.4)3(Ta0.5Zr0.5)2O12 optimal phosphor showed quantum yields of ∼40 and 48% under 393 and 463 nm excitations, respectively, a fluorescence lifetime of ∼0.66 ms for its main emission at 610 nm, color coordinates of around (0.653, 0.347), and can retain as high as ∼85% of its room-temperature emission intensity at 423 K. The phosphor also exhibited a favorable performance for n-UV excited LED lighting.