In order to find the Infinity Blade 2 Vile Helm, you’ll first have to get the Crown Map. You can get this map by an in store purchase for 175,000, or by finding it as a drop, your choice. Depends on if you have the money or not (tip: earn more money with these cheats).
After this you enter the cavern that is below the castle where you fight the Stone Demon. After fighting with the Titan on the bridge you’ll go forward and tap on the part of the carving on the opposite wall that the map tells you to. This will grant you the Vile Helm that you seek.
The helm itself is actually part of the entire Vile Set (the Blade, Armor, and Shield being the other primary items), which is needed to get the Holy Band. The set itself is geared towards attacking and speed instead of being able to boost your overall health and defense. Definitely an attacking set as opposed to a defensive set.