There is a product called self-levelling cement. It's quite liquid when
mixed properly and will spread-out like pancake batter and self-level
itself. Available at HomeDepot I think.

As far as the basement drain goes..It's intended to provide drainage
under the concrete floor to the gravel level underneath the concrete in
case your ho****er tank or other water devices burst a big water leak.
If you fill-it in with cement or grout and block it..you'll be sorry if
you flood your basement for whatever reason...your insurance adjuster
will see this in a heartbeat and deny an insurance claim on that basis.