  The complexity of interactions between the crystal-field and unusual
non-collinear spin arrangement in non-trivial magnets demands novel tools to
unravel the mystery underneath. In this work, we study such interaction
dynamics of crystal-field-excitations (CFE) and low-energy magnetic excitations
in orthochromite TmCrO3 with controls of temperature and magnetic field using
high-resolution magneto-terahertz (THz) time-domain spectroscopy. The THz
energy spectrum spanning 0.5-10 meV possesses a low-frequency spin-excitation
(magnon) mode and a multitude of CFE modes at 10 K, all of which uniquely
embody a range of phenomena. For the magnon mode, a temperature dependence of
peak frequency is induced by magnetic interactions between Tm and Cr
subsystems. While a change from blue- to red-shift of peak frequency of this
mode marks the magnetization reversal transition, the spin reorientation
temperature and change of magnetic anisotropy are depicted by different
features of field- and temperature-dependent peak frequency dynamics. The modes
corresponding to CFE are robust and laden with a multitude of sub-modes which
are attributes of non-trivial interactions across different transitions. These
modes are suppressed only upon substitution of Tb3+ at Tm3+ site, which
suggests a dominant role of single-ion anisotropy in controlling entire THz
excitations spectra. Overall, this remarkable range of phenomena seen through
the unique lens of all-optical THz tools provides deeper insights into the
origin of magnetic phases in systems with complex interactions between
rare-earth and transition metal ions and provides a multitude of a novel
combination of closely spaced modes for emerging hybrid spin-wave computation.
