You mean when you get 15 kills?
Yes you will only get the materials necessary to craft the armour. To craft the armor it takes 200k gold and 2 days. But it's totally worth it! Even if you don't decide to use it you can still level it and try fusing it. You will have a higher chance of getting good/rare armor than with other armors.
When you are battling epic bosses, to make them appear you have to fight minions correct? Do you have to fight minions on the current stage you are on (for example I'm on Epic Stage of Writhing Cascades) or can you just fight minions from other easier stages (like Relic Ruins or Thieves Cove)?