Apep is a rare monster who apears randomly in Steven's trading post and in Jigsaw pincer's coast.
Info about the anouncements of the Spawning of Apep:
If the player is in the field at the time when Apep spawns he will get a notice.
However if the Apep is already in the field when the player arrives, the player will Not get a notice.
If the player is in the field at the time when Apep dies he will also get a notice
Places where he is reported to spawn.
Steven's trading post:
Jigsaw Pincer's Coast: