The densification of nonoxide ceramics has been a known challenge in the field of engineering ceramics. The amount and type of sinter-aid together with sintering conditions significantly influence the densification behavior and microstructure in nonoxide ceramics. In this perspective, the present work reports the use of Eu2O3 sinter-aid and spark plasma sintering towards the densification of B4C. The densification is largely influenced by the solid-state sintering reactions during heating to 1900 degrees C. Based on the careful analysis of the heat-treated powder mixture (B4C-Eu2O3) and sintered compacts, the competitive reaction pathways are proposed to rationalize the formation of EuB6 as dominant microstructural phase. An array of distinctive morphological features, including intragranular and intergranular EuB6 phase as well as characteristic defect structures (asymmetric twins, stacking faults and threaded dislocations) are observed within dense B4C matrix. An attempt has been made to explain the competition between microstructure development and densification.