It's a suspense for now. I will write about how they were related in my 7th or 8th chapter. Please read my work and give me comments on where I am lacking. Thank you for comments and replies.

Sasuke is in control, and Indra left some energy/ chakra/will in Sasuke out of pity and left to hunt the God who wrote there fate like that out of anger.

precisely It was Indra who defeated them. Mostly correct.

After Sasuke awakened the Rinnegan, the Sage of Six Paths referred to him as the reincarnation of Indra similarly when he was explaining to others, he referred Madara as Indra's reincarnation, while he was alive and Madara awakened Rinnegan when he was old near to his end. Following the final battle between Naruto and Sasuke, they broke the reincarnation cycle. Still Sasuke is considered as reincarnation of Indra till he die.