The heavy atom effect on the magnetic anisotropy of septet trinitrenes is reported. Septet 1-bromo-3,5-dichloro-2,4,6-trinitrenobenzene (S-1) was generated in a solid argon matrix by ultraviolet irradiation of 1,3,5-triazido-2-bromo-4,6-dichlorobenzene. This trinitrene displays an electron spin resonance (ESR) spectrum that drastically differs from ESR spectra of all previously studied septet trinitrenes. The zero-field splitting (ZFS) parameters, derived from the experimental spectrum, show the parameter vertical bar D vertical bar = 0.1237 cm(-1) and the unprecedentedly large ratio of E/D = 0.262 that is close to the rhombic limit E/D = 1/3 for high-spin molecules. The CASCI (based on state-averaged CASSCF) and DFT methods were applied to calculate the ZFS tensor focusing on the heavy (bromine) atom effects on the spin-orbit term. These calculations show that the multiconfigurational ab initio formalism and the CASCI method are the most successful for accurate predictions of the spin-orbit term in the ZFS tensor of high-spin nitrenes containing heavy bromine atoms. Due to the presence of the bromine atom in S-1, the contribution of the spin-orbit term to the total parameter D is dominant and responsible for the unusual orientation of the easy Z-axis lying in the molecular plane perpendicular to the C-Br bond. As a result, the principal values D-XX, D-YY, and D-ZZ of the total tensor (D) over cap (Tot) have such magnitudes and signs for which the ratio E/D is close to the rhombic limit, and the total parameter D is large in magnitude and positive in sign.