The intensity of a nonlinear terahertz (THz) source is primarily given by its spectral density. In this letter, we introduce triangular Selinium (Se) as a novel THz emitter and show numerically its superior properties to the currently used crystals for intense THz generation. The excellent phase matching enables the applicability of elongated Se crystals which results in very high spectral flatness and broad THz bandwidth (0.5-3.5 THz), high conversion efficiency and THz pulse energy. The spectral THz density produced by optical rectification in Selenium exceeds those from contemporary crystal-based THz sources.