Diablo – What are these different Infernal Machines from the Level 12 Infernal Machine Plan

This post has been updated for 2.0.1 to only contain the locations of the keywardens.

The updated drop rates can be found in this answer:

Key of Destruction

• Odeg the Keywarden is in Fields of Misery in act 1. Odeg always spawns with the molten effect, and he has a big fireball spell that can do ok damage. He also has the jail affix and likes to kite a lot, making it difficult to engage him as a melee character.

Key of Hate

• Sokahr the Keywarden spawns in Dahlgur Oasis. Dahlgur oasis is quite big and he can spawn anywhere in that zone, so having multiple people helps a lot. Sokahr has an electric attack and a whirlwind attack. When he starts whirlwinding, I recommend to back up since that attack hurts a lot. He also has mortars.

• Another note here is that he is a cake walk for a demon hunter using the ball lightning attack. Since he often (always?) appears with a slow missile shield these balls hang around sparking on him if you can stay far enough away that he isn't chasing you. Alternately make him chase you in a circle so the balls stay within his shield.

Key of Terror

• Xah'Rith the Keywarden is in Stonefort. Xah'Rith is the hardest of the 3 key wardens as he teleports around a lot and have a high damage aoe attack. If you are a melee character, it helps a lot if you lure him to one of the many stair cases in the area so he can't run away as easily.

Infernal Machine Blacksmithing Plans

• Dropped from Nekarat the Keywarden in Act 4. Has very high damage and aoe. He also has a chance to drop any of the 3 keys.

The other answers both provide useful information, but I don't feel they really answer the question fully.

After wasting an Infernal Machine to attempt to solo it, asking around on the forums, and going through the fights with a wizard capable of soloing them on MP6, I feel I now have enough of a handle on it to answer.

Short version: There are certain gear/build combinations available (presumably for each class, although I can't say for certain) that make soloing absolutely feasible. However, just because you can solo all 4 Inferno acts at a given MP level effectively does not mean you can solo the uber bosses.

Many farming builds rely upon burning through enemies quickly and efficiently, and may relegate healing or resource regeneration to health globes or time. These builds still work with regular bosses because those encounters were designed to allow you to learn the mechanics enough to avoid or mitigate most of the major sources of incoming damages (i.e. dodging the butcher's chain attacks, avoiding the ground attacks during the appropriate phases of Belial, Azmodan, or Diablo, etc.).

This is much less feasible for the ubers, due to the combined attacks.

Therefore, in order to be able to solo, you need to have some form of sustained self-healing available (either high levels of life steal or stacked Life on Hit with a properly synergistic skill). Interrupting crowd controls will help a lot, but aren't as mandatory. Uber Siegebreaker, in particular, makes some form of healing on hit of utmost importance, even when grouping.

Without this, it isn't feasible to solo the encounters.

Even with the appropriate levels of survivability, you need to have sufficient DPS, as not only are enrage timers an issue, but you have to worry about keeping the NV stacks up. If you take too long, particularly if it takes you multiple attempts, you will have to periodically go out and hunt elites to refresh your stacks, lest you risk losing your stacks partway through an uber fight (and thereby guaranteeing that they will not drop the organs you seek).