Questions about Ostius

It’s said at one point, I think by Cristoval, that Ostius can only travel between Res and Luceum.

First, how did he get this ability? He isn’t synchronous, otherwise he wouldn’t need Vis. But this seems incredibly powerful.

Second, can he not travel to Obiteum, or does he just choose not to because “Obiteum is lost”. This seems consistent with other theories that the Concurrence won on Obetium.

Third, wtf is this dudes actual goal??