The plans for the Infernal Machine can be dropped by Nekarat the Keywarden in first extent of the Silver Spire in Act IV.
Nekarat is very pretty impossible to kill but luckily you only need him to drop the plansonce.
The curse reduces the effectiveness it’s a perfect idea to fight him with a number of your friends on the highest Monster Power you think you can handle. Now pay attention please. By handle, Know what, I mean the Monster Power that will cause you and your friends to die less than 10 times every before you manage to kill him. Then again, he attacks with a fiery spell that quickly drains your health, he has the jailer affix and he can curse you. He also will use his whirlwind blade attack on you. Let me tell you something. Sokahr has the missle dampening, vortexand mortar affixes. Now regarding the aforementioned fact… The Key of Hate can be dropped by Sokahr the Keywarden in the Dahlgur Oasis in Act I. Just think for a moment. The Key of Destruction can be dropped by Odeg the Keywarden in the Fields of Misery in Act Odeg’s attacks include 3 simultaneous fire balls, a spear attack, he has the Molten affix and he is able to Jail you.
The Key of Terror can be dropped by Xah’Rith the Keywarden in Stonefort in Act II.
He always appears in the Raise the Catapults section of Stonefort which means you can use the Stonefort waypoint and head to the right and up the stairs.
In order to create a Infernal Machine, you need the plans for the Infernal Machine and you must combine 3 keys which are dropped by special monsters that are located in ActsI through II.