Q: How do I use the monster power system?
A: First, you need to enable it in the 'Gameplay' page in the options menu. There is a checkbox labeled "Enable Monster Power Selection." Once this box is checked, you can select a monster power level in the bottom-left-side of the quest selection screen from the menu. NOTE Monster power must be set before you start a game, and cannot be changed while you are in a game.
Q: What are the rewards for playing with monster power on?
A: With monster power enabled, you are granted scaling bonuses to magic find, gold find and experience gain, as well as (on inferno) the chance for a bonus item to drop from any monster that has passed the initial roll drop an item. The full rewards of the monster power system are outlined in the blog, found here:
One last benefit of monster power is that all monsters are set to mlvl 63 when playing on monster power 1 or higher. With the change to affix rolls in 1.0.5, this means that any item below ilvl 63 will still roll affixes as if it were an ilvl 63 item.
Q: Where is the best place to farm?
A: Anywhere you want! The monster power system makes it so that monsters across all acts of the game have roughly the same damage and HP. IE: Monsters in act 1 on monster power 1 will have the same HP and damage values as monsters in act 3 on monster power 1.
The Infernal Machine
Q: Where are the Keywardens?
A: The Act 1 warden is located in the Fields of Misery. The act 2 warden is located in Dahlgur Oasis. The act 3 warden is located in Stonefort. The act 4 warden is located in The Silver Spire, Level 1. The first 3 wardens drop keys which will be needed to craft each infernal machine, while the Act 4 warden drops the plans for crafting the Infernal Machine. You will need to find one of each key for every infernal machine you make, but you only need to find the plans once.
Q: What are the chances for the keys to drop? Does magic find affect them? How about nephalem valor?
A: They keywardens will only drop the keys on Inferno difficulty, and only if you have 5 stacks of nephalem valor when you kill them. Without monster power enabled (aka MP0), there is a 5% chance for a warden to drop a key when killed. Each successive difficulty has a 10% chance per monster power level; 10% chance on monster power 1, 50% on monster power 5, 100% chance on monster power 10.
Magic find does not affect these chances. These chances are identical for the organs dropping from 'Uber Bosses.'
Q: How many keys do I need?
A: Each Infernal Machine requires 1 of each key to be made, and is consumed on use. To make a Hellfire ring, you need one each of 3 organs. Assuming each 'uber boss' you kill drops his organ, you will need 3 of each key to make 1 hellfire ring. You only have to find the plans for the Infernal Machine once, however. They keys do not stack, so its wise to create the infernal machines to save storage space.
Q: Can I get multiple copes of the same key? or multiple Infernal Machines?
A: Yes. In fact, it's probably a good idea to wait until you have 3 machines before attempting the 'ubers.' When a machine is used, the portal it opens is random. However, each portal can only be opened once in each game, so you will be guaranteed to get all 3 portals if you use 3 machines in the same game.
Q: Can I play with other people in monster power games?
A: Yes, but only private games. The monster power system is unavailable in public games.
Q: Where do I find the jewelcrafting and blacksmithing plans I will need?
A: The blacksmithing plans for the infernal machine can be found on the Act 4 key warden in The Silver Spire, level 1. You will need 5 stacks of NV to see the drop.
The jewelcrafting plans for the hellfire ring can be bought for 2 million gold in inferno act 2 from squirt the peddler, the small girl NPC just north of the waypoint. Buying the plans once will teach your jewelcrafter how to make all 4 versions of the ring.
These plans both count towards the achievements to teach your crafting npc a certain number of plans.
Q: Where do I use the Infernal Machine?
A: There is a shack behind the healer in New Tristram in act 1. When you are on the quest "Return to New Tristram," the wooden door on this shack can be broken.
Q: What is so special about the Hellfire ring? What if i don't like mine?
A: The Hellfire ring grants a bonus 35% experience and a chance to launch a fireball when you attack. When you craft the ring, you select one of the 4 core stats, which the ring will receive 170-200 of. The ring also spawns with 4 random modifiers.
If you aren't happy with your ring, it doesn't have a level requirement, so its perfect for leveling an alt. If you have absolutely nothing to do with your ring, it can be salvaged for inferno level crafting mats.
This is by no means a comprehensive list, at least not yet. It's very late, so I'm going to go to sleep, and I will update this with more FAQs tomorrow. Feel free to drop your questions or any questions you've seen clogging up the front page, and let me know if / where I've made any mistakes.