Sated devourer is the final item form of the devourer enchantment. You can get sated by getting 30 stacks of devourer. Build for devourer: 
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. Once you have the devourer you can stack it up. You can get stacks from killing jungle monsters, getting kills or assists on champion kills, killing scuttle crab, and taking down epic monsters such as dragon and baron.
The item starts of with 30 magic damage on-hit. Each stack you get adds magic damage to the item. Max of 30 stacks. So with sated you will have 60 magic damage every auto. Sated, like I said before, is evolved devourer. Once you get 30 stacks on devourer it evolves into sated.
Sated does not make you auto attack twice. Every other auto attack, on hit effects will apply twice.
So if you know of Gnar, his w allows every third auto attack to do percentage health damage. With sated, he would apply one stack on the first auto, then he would apply two stacks on the second auto. ( auto = auto attack ). Once again sated makes every other auto apply on hit effects twice.
Why get this item over others? Well it depends. I use devourer/sated on late game champions like Master Yi, or Aatrox. With devourer it makes my Aatrox be some what weak early game, strong mid game because of sated, and very strong late game. I've seen sated used on Warwick, Shyvanna, Shen, Master Yi, and some others. What do these have in common? They all have an on hit effect that will benefit from the sated's passive ( every other auto attack applies on hit effects twice ).
It wasn't exactly short but I tried to put it in a simple form to understand. If you're still confused Google some YouTube videos about it.
