The exploration of the QCD phase diagram (T-$\mu_{B}$) particularly the search for a phase transition from hadronic to partonic degrees of freedom and the critical point is one of the most challenging theoretical and experimental tasks in present heavy ion physics. Unfortunately the QCD predictions are to a large extent qualitative, as QCD phenomenology at finite temperature and baryon number is one of the least explored domains of the theory.

The NA49 study of central collisions of heavy nuclei at CERN SPS energies revealed rapid changes in the energy dependence of hadron production properties in the narrow range arround 30A GeV. The results indicate the onset of deconfinement in the early stage of the produced system and motivate similar studies by the NA61/SHINE experiment and the ongoing search for the predicted critical point. NA61/SHINE results on particle production properties as well as event-by-event fluctuations in p+p, Be+Be and Ar+Sc collisions are shown as a function of beam energy and system size. The observation of rapid change of hadron production properties that starts when moving from light to intermediate mass ion collisions might be interpreted as the beginning of the formation of large clusters of strongly interacting matter - the onset of fireball. The future ion program of the NA61/SHINE experiment including planned measurements of charm hadrons, mostly D mesons, production in Pb+Pb collisions is presented.