Magnetic van der Waals materials represent a fascinating field of condensed matter physics. Here we will focus on a group of transition metals trihalides, which allow for a true 2D magnetism present in a monolayer. This means a strong application potential, but knowledge of magnetic properties of the trihalides is at the very beginning, in many cases there is no consensus about its crystal structure, which reveals signatures of dimorphism. The first studies point out to strong coupling between bonds length and angles and magnetic structure. Our project combines rich experience with single crystal preparation, unique possibilities of studies in various external conditions (namely wide interval of temperatures, magnetic fields and pressures) and ab initio calculations. We will study parameters and symmetry of the magnetocrystalline anisotropy, and ways to increase the limited temperature range of ferromagnetic order here. Pressure experiments are very effective to reveal the effect of structure parameters on the inter- and intra-layer magnetic interactions.