  In van der Waals (vdW) materials, strong coupling between different degrees
of freedom can hybridize elementary excitations into bound states with mixed
character. Correctly identifying the nature and composition of these bound
states is key to understanding their ground state properties and excitation
spectra. Here, we use ultrafast spectroscopy to reveal bound states of
d-orbitals and phonons in 2D vdW antiferromagnet NiPS3. These bound states
manifest themselves through equally spaced phonon replicas in frequency domain.
These states are optically dark above the N\'eel temperature and become
accessible with magnetic order. By launching this phonon and spectrally
tracking its amplitude, we establish the electronic origin of bound states as
localized d-d excitations. Our data directly yield electron-phonon coupling
strength which exceeds the highest known value in 2D systems. These results
demonstrate NiPS3 as a platform to study strong interactions between spins,
orbitals and lattice, and open pathways to coherent control of 2D magnets.
