Mysterium rhymes with:
deuterium, deuterium, aquarium, atrium, auditorium, bacterium, barium, crematorium, delirium, deuterium, disequilibrium, emporium, equilibrium, herbarium, honorarium, merriam, miriam, moratorium, myriam, opprobrium, planetarium, tellurium, thorium, yttrium
Mysterium sounds like:
mainstream, masterman, mastrianni, mastriano, mastroianni, mcdearmon, mcdorman, mcdurman, mcternan, mctiernan, minestrone, mostrom, munsterman