  We present the luminosity function (LF) for ultraluminous Ly$\alpha$ emitting
galaxies (LAEs) at z = 6.6. We define ultraluminous LAEs (ULLAEs) as galaxies
with logL(Ly$\alpha$) > 43.5 erg s$^{-1}$. We select our main sample using the
g', r', i', z', and NB921 observations of a wide-area (30 deg$^2$) Hyper
Suprime-Cam survey of the North Ecliptic Pole (NEP) field. We select candidates
with g', r', i' > 26, NB921 $\leq$ 23.5, and NB921 - z' $\leq$ 1.3. Using the
DEIMOS spectrograph on Keck II, we confirm 9 of our 14 candidates as ULLAEs at
z = 6.6 and the remaining 5 as an AGN at z = 6.6, two [OIII]$\lambda$5007
emitting galaxies at z = 0.84 and z = 0.85, and two non-detections. This
emphasizes the need for full spectroscopic follow-up to determine accurate LFs.
In constructing the ULLAE LF at z = 6.6, we combine our 9 NEP ULLAEs with two
previously discovered and confirmed ULLAEs in the COSMOS field: CR7 and COLA1.
We apply rigorous corrections for incompleteness based on simulations. We
compare our ULLAE LF at z = 6.6 with LFs at z = 5.7 and z = 6.6 from the
literature. Our data reject some previous LF normalizations and power law
indices, but they are broadly consistent with others. Indeed, a comparative
analysis of the different literature LFs suggests that none is fully consistent
with any of the others, making it critical to determine the evolution from z =
5.7 to z = 6.6 using LFs constructed in exactly the same way at both redshifts.
