The fabrication of dense amorphous Si-B-C-N monoliths is a processing challenge given that it is hard to avoid crystallization at the sintering temperatures needed to attain full density up to 1900 degrees C for conventional hot pressing and SPS methods. We report here successful densification of amorphous Si2BC3N monoliths achieved by heating at 1100 degrees C and 5 GPa. The relationships between microstructure, types of chemical bonding, and mechanical properties were investigated. The strong amorphous 3-D networks of Si-C, C-B, C-N (sp(3)), N-B (sp(3)), and C-B-N bonds provide high densities at high applied pressure and thus amorphous Si2BC3N monoliths show high hardness of 29.4 GPa and elastic modulus of 291 GPa. The amorphous structure is lost with crystallization of beta-SiC and BN(C) reducing contributions from Si-C, C-N (sp(3)), and C-B-N bond networks thereby decreasing mechanical properties.