In the Achievements section under Games, there's these achievements you can get if you "Find the # missing page for Alexander!". Does anyone know what game these achievements come from?

They were from a plot and are now retired. I don't remember which plot/event they were for now though, sorry :c

Aw man, bummer. I really wanted one of those achievement prizes. Thank you for answering my question!

Sorry to be the bearer of bad news :c But you're welcome~

Oh that's quite alright, no worries :)