jats:p<jats:styled-content style="fixed-case">XMM</jats:styled-content>‐Newton is capable of making a transformational advance in our understanding of how luminous accreting black holes work, by dedicating about 10% of future observing time to long observations, of order megaseconds, to X‐ray variable active galactic nuclei (<jats:styled-content style="fixed-case">AGN</jats:styled-content>) research. This would enable reverberation studies, already a commonplace feature of <jats:styled-content style="fixed-case">AGN</jats:styled-content>, to proceed to the next level and follow the behavior of the powerful dynamic corona. Such a dedicated legacy program can only be carried out with <jats:styled-content style="fixed-case">XMM</jats:styled-content>‐Newton.</jats:p>