  We prove the existence and uniqueness of a new maximal subgroup of the
algebraic group of type $E_8$ in characteristic $3$. This has type $F_4$, and
was missing from previous lists of maximal subgroups produced by Seitz and
Liebeck--Seitz. We also prove a result about the finite group $H={}^3\!D_4(2)$,
that if $H$ embeds in $E_8$ (in any characteristic $p$) and has two composition
factors on the adjoint module then $p=3$ and $H$ lies in this new maximal $F_4$
subgroup.
