Need some help with my homebrew campaign plot.

BBEG (evil lich version of Alexander the Great) has already conquered the world and has grown bored of ruling his dominion. He wants to open a permanent portal to another world/dimension to continue conquering, and in so doing, destroy this world. The party's goal is to essentially build up the rebellion to defeat the BBEG before he can accomplish this goal. What are some interesting plot devices/mechanics I could use to A) introduce the BBEG's plans piecemeal to the party, and B) have them build alliances and secure resources to the rebel effort?