There exists exact solutions of Einstein's equations that describe the full non-linear dynamics of wave propagation in GR. While their perturbative limit is sufficient for comparison to observations in gravitational wave detectors, the fact that these solutions encapsulate the complete non-linear dynamics of wave propagation is very useful to probe advanced aspects of GR, such as the formation of apparent horizons by colliding plane waves or non-linear memory effects. I present a non-linear gravitational wave solution of a Horndeski scalar-tensor theory. This solution is constructed by applying a disformal transformation to an existing seed solution that describes breathing gravitational waves generated by a scalar field. I study the properties of the resulting solution, and show that contrary to the seed it also contains tensorial gravitational waves. The sourcing of such waves by a monopolar scalar field, which is an intriguing result, is made possible by the higher-order nature of Horndeski gravity.
