A 'Sunday Special' Monster 25 is available on Sundays only, with an exceptionally powerful reward, higher than any other monster. The requirement to be able to hunt for this Monster is that you have used a certain amount of summoned warriors to create new legions over the last 6 days since Monday 0:01am UTC. Updating existing legions does NOT count towards the requirement. You must use the required amount of warriors to create NEW legions.
Only one legion will be able to attack this monster, and only once per week on Sundays. Monster 25 needs to be hunted individually, in other words the "mass hunt" option will not hunt for this monster.