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The nature and origin of supermassive black holes (SMBHs) remain an openmatter of debate within the scientific community. While various theoreticalscenarios have been proposed, each with specific observational signatures, thelack of sufficiently sensitive X-ray observations hinders the progress ofobservational tests. In this white paper, we present how AXIS will contributeto solving this issue. With an angular resolution of 1.5$^{\prime\prime}$on-axis and minimal off-axis degradation, we have designed a deep surveycapable of reaching flux limits in the [0.5-2] keV range of approximately2$\times$10$^{-18}$ \fcgs~ over an area of 0.13 deg$^2$ in approximately 7million seconds (7 Ms). Furthermore, we have planned an intermediate depthsurvey covering approximately 2 deg$^2$ and reaching flux limits of about2$\times$10$^{-17}$ \fcgs ~ in order to detect a significant number of SMBHswith X-ray luminosities (L$_X$) of approximately 10$^{42}$ \lx up to z$\sim$10.These observations will enable AXIS to detect SMBHs with masses smaller than10$^5$ \ms, assuming Eddington-limited accretion and a typical bolometriccorrection for Type II AGN. AXIS will provide valuable information on theseeding and population synthesis models of SMBH, allowing for more accurateconstraints on their initial mass function (IMF) and accretion history fromz$\sim$0-10. To accomplish this, AXIS will leverage the unique synergy ofsurvey telescopes such as JWST, Roman, Euclid, LSST, and the new generation of30m class telescopes. These instruments will provide optical identification andredshift measurements, while AXIS will discover the smoking gun of nuclearactivity, particularly in the case of highly obscured AGN or peculiar UVspectra as predicted and recently observed in the early Universe.

Surveying the onset and evolution of supermassive black holes at high-z with AXIS