In one issue during Mark Waid and Bryan Hitch's JLA Batman turned Slender Man
It happened in the story of the sixth dimensional creatures who turn peoples desire to reality, the leagures who showed their secret identities to the rest were separated from the mselves
Batman was separated from Bruce Wayne
Superman was removed from Clark Kent
Plastic Man was his former outlaw self no more
Jonn' Jo'nzz was separated from John Jones
Kyle Ryner and Green Lantern were seprated
Aquaman asked Batman for theories during a fight, and he was shocked to see a facelss Batman under the mask