This study presents a series of phosphors codoped with Cr³+ and Ni²+ that exhibit near-infrared-II (NIR-II) emission with an unprecedented internal quantum efficiency (IQE) of 97.4%. The research reveals an energy transfer mechanism involving Cr³+ clusters, where luminescent centers are closely matched in energy, and Ni²+ emission intensity can be tuned through sintering temperature. Electron paramagnetic resonance (EPR) studies show interactions between Cr³+ clusters and Cr³+–Ni²+ pairs, confirming the cause of high IQE and the significance of Cr³+ clusters. The work provides promising pathways for developing NIR-II light-emitting diodes (LEDs) with outstanding efficiency by suggesting a new energy transfer source of Cr³+. The study investigates energy transfer from single Cr³+ ions and Cr³+ clusters to Ni²+ in the Mg₁₋ᵧGa₂₋ₓO₄:xCr³+,yNi²+ series with an intermediate spinel-type structure. Detailed crystal structural analysis and luminescence properties are reported, along with energy transfer mechanisms. Structural analysis shows that MgGa₂O₄ has an intermediate spinel-type crystal structure with an Fd3m space group, comprising six-coordinate octahedra (M1) and four-coordinate tetrahedra (M2) in a 2:1 ratio. This structure can host codoped Cr³+ (substituting Ga³+) and Ni²+ (substituting Mg²+), forming Mg₁₋ₓGa₂₋ₓO₄:xCr³+,yNi²+. The photoluminescence (PL) properties of single-doped Cr³+ and Ni²+ in the MgGa₂O₄ structure were previously reported, and this study focuses on the codoped system. The PL spectrum of single-doped Cr³+ samples shows three bands: the R-band (708 nm), the N-band (750 nm), and the C-band (850 nm), arising from the emission of single Cr³+, Cr³+–Cr³+ pairs, and Cr³+ clusters, respectively. The IQE of the samples is listed in Table S5, where an IQE > 97% up to x = 0.06 is found. Based on this result, the x = 0.02 and x = 0.06 compositions are identified as low and high Cr³+ concentration groups for our Cr³+ to Ni²+ energy transfer research targeting the energy transfer mechanism from single Cr³+ and Cr³+ clusters. The photoluminescence excitation (PLE) spectra of both series Mg₁₋ᵧGa₂₋ₓO₄:xCr³+,yNi²+, x=0.02,y=0.005-0.04This study presents a series of phosphors codoped with Cr³+ and Ni²+ that exhibit near-infrared-II (NIR-II) emission with an unprecedented internal quantum efficiency (IQE) of 97.4%. The research reveals an energy transfer mechanism involving Cr³+ clusters, where luminescent centers are closely matched in energy, and Ni²+ emission intensity can be tuned through sintering temperature. Electron paramagnetic resonance (EPR) studies show interactions between Cr³+ clusters and Cr³+–Ni²+ pairs, confirming the cause of high IQE and the significance of Cr³+ clusters. The work provides promising pathways for developing NIR-II light-emitting diodes (LEDs) with outstanding efficiency by suggesting a new energy transfer source of Cr³+. The study investigates energy transfer from single Cr³+ ions and Cr³+ clusters to Ni²+ in the Mg₁₋ᵧGa₂₋ₓO₄:xCr³+,yNi²+ series with an intermediate spinel-type structure. Detailed crystal structural analysis and luminescence properties are reported, along with energy transfer mechanisms. Structural analysis shows that MgGa₂O₄ has an intermediate spinel-type crystal structure with an Fd3m space group, comprising six-coordinate octahedra (M1) and four-coordinate tetrahedra (M2) in a 2:1 ratio. This structure can host codoped Cr³+ (substituting Ga³+) and Ni²+ (substituting Mg²+), forming Mg₁₋ₓGa₂₋ₓO₄:xCr³+,yNi²+. The photoluminescence (PL) properties of single-doped Cr³+ and Ni²+ in the MgGa₂O₄ structure were previously reported, and this study focuses on the codoped system. The PL spectrum of single-doped Cr³+ samples shows three bands: the R-band (708 nm), the N-band (750 nm), and the C-band (850 nm), arising from the emission of single Cr³+, Cr³+–Cr³+ pairs, and Cr³+ clusters, respectively. The IQE of the samples is listed in Table S5, where an IQE > 97% up to x = 0.06 is found. Based on this result, the x = 0.02 and x = 0.06 compositions are identified as low and high Cr³+ concentration groups for our Cr³+ to Ni²+ energy transfer research targeting the energy transfer mechanism from single Cr³+ and Cr³+ clusters. The photoluminescence excitation (PLE) spectra of both series Mg₁₋ᵧGa₂₋ₓO₄:xCr³+,yNi²+, x=0.02,y=0.005-0.04