High-entropy materials (HEM) represent a new class of solid solutions containing at least five different elements. Their compositional diversity makes them promising as platforms for development of functional materials. We synthesized HEMs in a complex mullite-type structure challenging previous assumptions that HEMs only form in simple structures. We present five new HEMs i.e., Bi2(Al0.25Ga0.25Fe0.25Mn0.25)4O9, and A2Mn4O10 with variations of A = Nd, Sm, Y, Er, Eu, Ce, Bi demonstrating the vast accessible composition space. By combining scattering, microscopic, and spectroscopic techniques, we show that our materials are mixed solid solutions. Remarkably, when following their crystallization in situ using X-ray diffraction and X-ray absorption spectroscopy we find that the HEMs form through a metastable amorphous phase without the formation of any crystalline intermediates. We believe that our synthetic route is excellently suited to synthesize diverse HEMs and therefore will have a significant impact on their future exploration.