{{REVISIONUSER}} is a "magic word" that returns the last person to change an article. On November 14, 2013, this feature was expanded to include {{REVISIONUSER:(article-name)}}, which makes it possible to query who edited any of a number of other articles. This template does not need to be used in place of the magic word normally, but is provided so that a WP:Lua module can use frame:expandTemplate to access this function. (There may well be some other better way to do this).
Example: {{REVISIONUSER|Astronomy}} -> Athrante. (i.e. User:Athrante)
But you can get this straight from the parser function by typing {{REVISIONUSER:Astronomy}}.
{{REVISIONUSER}}
magic word (tracked in Phabricator as Task T166946) which prevents it from working properly if the user name starts with certain wiki-markup characters such as a * and possible other characters. The work-around is to wrap the call to the magic word with the nowiki function of Module:String2:
{{#invoke:String2|nowiki|{{REVISIONUSER}}}}
->{{#invoke:String2|nowiki|{{REVISIONUSER:User:***SkooterBecca***}}}}
-> ***SkooterBecca***{{#invoke:String2|nowiki|{{REVISIONUSER|User:***SkooterBecca***}}}}
-> ***SkooterBecca***