A direct dearomative [4 + 2] annulation of electron-poor N-heteroarenes with azoalkenes generated in situ from α-halogeno hydrazones was developed under mild conditions. Accordingly, a series of fused polycyclic tetrahydro-1,2,4-triazines with potential biological activity were obtained in up to 96% yield. Various α-halogeno hydrazones and N-heteroarenes, such as pyridines, quinolines, isoquinolines, phenanthridine, and benzothiazole, were tolerated by this reaction. The general applicability of this method was shown by upscale synthesis and product derivatization.