Almost all technologically relevant solidification is based on heterogeneous nucleation. While it is known that the presence of impurities enhances nucleation processes and influences the evolution of the microstructure, little is known about the initial stages of nucleation from an atomic scale. This is because nucleation time scales are too long for conventional molecular dynamics methods and therefore a ’rare event’. We employ transition path sampling, an atomistic simulation method, to study the effect of prestructured seeds on nucleation in nickel. We use seeds of varying sizes and crystal structures and quantify their effect on the rate constant and free energy barrier to understand the role of seeds in nucleation mechanism. Even small seeds are capable of inducing spontaneous nucleation. The nucleation rate is controlled by the size and surface geometry of the seed. Moreover, the structure of the seed determines the structure of the solid crystal. The seeds facilitate pre-ordering in the liquid and different seeds may be used to induce the formation of specific polymorphs.