What kind of tool will loosen this nut holding a chainwheel on a one piece crank?
Such a tool may be called hook spanner.

The nut is holding the inner race of the bottom bracket bearing.

A web search with "bottom bracket" and "hook spanner" produces a couple of hits. (For instance, the tool mentioned by Klaster in the comments.) You may use a vernier caliper to determine the dimensions of your nut and find the right tool for you.