Superman is in Kandor in Action 734 (June 1997), by Michelinie, Grummett and Rodier.
The Atom has accompanied him. Ray Palmer was de-aged by Extant, so this is the young and insecure Atom. They find that Kandor’s society has broken down, since Tolos was removed as their leader. There are still forces loyal to him, who are after Superman. Superman and the Atom join with Faern, Scorn’s sister.
Back in Metropolis, Scorn is trying to take Superman’s place, and captures a new villain, the Rock, stopping another of his attempts to kill Luthor.
I love that, after all the changes Superman has gone through, the MSCU aren’t sure whether this is really him or not.
Superman gets captured by those loyal to Tolos.
Faern bluffs the guy into releasing Superman, telling him the Atom has entered his system, and will blwo him up from inside if he doesn’t. That works.