We all know that Tobi (Obito) has a Mangekyou Sharingan and it is the same as Kakashis. However we don't know how he unlocked it. From what we know you will need to kill your best friend in order to get it, we also know that Madara and Obito knew each other before Madara died. So what I'm thinking is that Madara saves Obito and trains him, when Madara sees Obito as strong enough to follow up with the plan without Madara.. Madara asks Obito to kill him so that Obito will get the Mangekyou Sharingan and so that Madara could be revived later when he was needed. The only hole in this is that Madara wouldn't want someone to kill him like that unless he had a really good reason. Maybe Madara was sick and would have died anyways so in order for his plan to work and so that he could live longer he made Obito kill him so that Obito could become strong enough to carry out the plan and then revive him. Because we all know that Madara mostly care about himself... 