  The luminosity function (LF) of active galactic nuclei (AGN) probes the
history of supermassive black hole assembly and growth across cosmic time. To
mitigate selection biases, we present a consistent analysis of the AGN LFs
derived for both X-ray and mid-infrared (MIR) selected AGN in the XMM-Large
Scale Structure (XMM-LSS) field. There are 4268 AGN used to construct the MIR
luminosity function (IRLF) and 3427 AGN used to construct the X-ray luminosity
function (XLF), providing the largest census of the AGN population out to $z=4$
in both bands with significant reduction in uncertainties. We are able for the
first time to see the knee of the IRLF at $z>2$ and observe a flattening of the
faint-end slope as redshift increases. The bolometric luminosity density, a
proxy for the cosmic black hole accretion history, computed from our LFs shows
a peak at $z\approx2.25$ consistent with recent estimates of the peak in the
star formation rate density (SFRD). However, at earlier epochs, the AGN
luminosity density is flatter than the SFRD. If confirmed, this result suggests
that the build up of black hole mass outpaces the growth of stellar mass in
high mass systems at $z\gtrsim 2.5$. This is consistent with observations of
redshift $z\sim 6$ quasars which lie above the local $M-\sigma$ relationship.
The luminosity density derived from the IRLF is higher than that from the XLF
at all redshifts. This is consistent with the dominant role of obscured AGN
activity in the cosmic growth of supermassive black holes.
