  We utilize deep near-infrared survey data from the UltraVISTA fourth data
release (DR4) and the VIDEO survey, in combination with overlapping optical and
Spitzer data, to search for bright star-forming galaxies at $z \gtrsim 7.5$.
Using a full photometric redshift fitting analysis applied to the $\sim 6\,{\rm
deg}^2$ of imaging searched, we find 27 Lyman-break galaxies (LBGs), including
20 new sources, with best-fitting photometric redshifts in the range $7.4 < z <
9.1$. From this sample we derive the rest-frame UV luminosity function (LF) at
$z = 8$ and $z = 9$ out to extremely bright UV magnitudes ($M_{\rm UV} \simeq
-23$) for the first time. We find an excess in the number density of bright
galaxies in comparison to the typically assumed Schechter functional form
derived from fainter samples. Combined with previous studies at lower redshift,
our results show that there is little evolution in the number density of very
bright ($M_{\rm UV} \sim -23$) LBGs between $z \simeq 5$ and $z\simeq 9$. The
tentative detection of an LBG with best-fit photometric redshift of $z = 10.9
\pm 1.0$ in our data is consistent with the derived evolution. We show that a
double power-law fit with a brightening characteristic magnitude ($\Delta
M^*/\Delta z \simeq -0.5$) and a steadily steepening bright-end slope ($\Delta
\beta/\Delta z \simeq -0.5$) provides a good description of the $z > 5$ data
over a wide range in absolute UV magnitude ($-23 < M_{\rm UV} < -17$). We
postulate that the observed evolution can be explained by a lack of mass
quenching at very high redshifts in combination with increasing dust
obscuration within the first $\sim 1 \,{\rm Gyr}$ of galaxy evolution.
