From Diablo Wiki
Key Run refers to a game played in order to obtain keys for the Infernal Machine event, from one of the three Keywardens. Keys only drop on Inferno difficulty to characters with five stacks of Nephalem Valor, so it's essential that players stack up before killing the keywarden.
Keys drop more frequently on higher levels of Monster Power, thus players seeking keys should play on the highest MP level they can handle. It's also recommended that players do key runs in multiplayer games, since this makes the process go more quickly, and each character has the same odds for a key to drop whether playing solo or in a party.
 Key Drop Odds
Higher levels of monster power improve the chances of a key dropping. Only Monster Power affects this; Magic Find is not a factor. (Though a character's Magic Find does influence the other items dropped by the Keywarden.) Keys drop only on Inferno and only for players with 5 stacks.
- MP0 = 5% chance of a key dropping
- MP1 = 10% chance of a key dropping
- MP2 = 20% chance of a key dropping
- MP3 = 30% chance of a key dropping
- MP4 = 40% chance of a key dropping
- MP5 = 50% chance of a key dropping
- MP6 = 60% chance of a key dropping
- MP7 = 70% chance of a key dropping
- MP8 = 80% chance of a key dropping
- MP9 = 90% chance of a key dropping
- MP10 = 100% chance of a key dropping
These are the odds for a key drop to each player in the game, and more than one key can drop when there are multiple players present.
 Keywarden Locations
The three keywardens and the planwarden are:
- The Key of Destruction will drop from Odeg the Keywarden in Act I - Fields of Misery
- The Key of Hate will drop from Sokahr the Keywarden in Act II - Dahlgur Oasis
- The Key of Terror will drop from Xah'Rith the Keywarden in Act III - Stonefort
- The Blacksmithing Plans will drop from Nekarat the Keywarden in Act IV - Silver Spire Level 1
 Multiplayer Strategy Efficiency
Not only does more players make finding and killing the keywardens quicker and easier, but every player in the game has the same chance to see a key drop. This makes it much more likely that at least one key will drop, and it's quite possible that more than one will fall for different players.
For example, a 4 player game on MP10 would yield a guaranteed key for each player. Thus if the same group of 3 players did all three keywardens in consecutive games, each character would have 1 of each key, which would let each player create a portal device.
The ideal system would be for players to do keyruns on Act 2 and Act 3 in two consecutive games. Then in the third game they'd do a keyrun in Act One, which would give them each one of each key. They'd go straight from there into the Hermit's Shack and open the three portals in sequence, giving them each enough Demonic Organs for a Hellfire Ring, in just 3 quick games.
The four keywardens in their natural environments.
|Key of Destruction||Key of Hate|
|Odeg in the Fields of Misery, Act 1||Sokahr in the Dahlgur Oasis, Act 2|
|Key of Terror||Infernal Machine Plan|
|Xah'Rith on the Stonefort, Act 3||Nekaret in Silver Spire, level 1, Act 4|