can anyone tell me what to expect when Overmind sends attacking fleet to my planet ?

beter dont aid the overlord
overmind will reward you with devourers
if you aid overlord overmind will attack you

what kind of force he sends? how many creatures and what  kind ?

he sends 20-27 devrouers to attack but they leave no debris even if you manage to kill them 

THNX!

if you want a alliance you can join United Nation we are the only large english speaking alliance