  Rare-earth intermetallic compounds exhibit rich phenomena induced by the
interplay between localized $f$ orbitals and conduction electrons. However,
since the energy scale of the crystal-electric-field splitting is only a few
millielectronvolts, the nature of the mobile electrons accompanied by
collective crystal-electric-field excitations has not been unveiled. Here, we
examine the low-energy electronic structures of CeSb through the anomalous
magnetostructural transitions below the N$\'{e}$el temperature, $\sim$17 K,
termed the 'devil's staircase', using laser angle-resolved photoemission, Raman
and neutron scattering spectroscopies. We report another type of electron-boson
coupling between mobile electrons and quadrupole crystal-electric-field
excitations of the 4$f$ orbitals, which renormalizes the Sb 5$p$ band
prominently, yielding a kink at a very low energy ($\sim$7 meV). This coupling
strength is strong and exhibits anomalous step-like enhancement during the
devil's staircase transition, unveiling a new type of quasiparticle, named the
'multipole polaron', comprising a mobile electron dressed with a cloud of the
quadrupole crystal-electric-field polarization.
