What Satan says in the first chapter is basically:
Let's go to Gehenna.
Will have to destroy that thing [Kanji: sword] to free you from this seal/curse.
Regain your power [Kanji: true nature] as a demon.
I'm a perfect entity who reigns_over/controls/dominates/unifies all powers/forces/authorities.
But even someone sush as me, have one little flaw/downside. Do you get what that is?
That is, here, in Assiah, there is no equivalent matter.
The matter affected by me immediately breaks.
I want Assiah.
You exist in Assiah while being a god of gehenna; while drawing my blood [Kanji: flames]. ["drawing blood" means "inheriting from"/"being the descendant of"]
In order for me to obtain Assiah, you are indispensable.
Today you will be reborn.
In other words:
I want to exist in Assiah but everything I possess burns.
That's why I need you. You can exist in Assiha and you are powerful enough to resist my flames.
So let's go to gehenna and make you regain your demon power/nature so I can possess you and go to Assiah.
or
I want to rule over Assiah but everything I possess burns.
That's why I need you. You can exist in Assiha and you are powerful enough to rule it.
So let's go to gehenna and make you regain your demon power/nature so I can make you my representative and send you back to Assiah.
With the first interpretation, we don't know what Satan needs to do to possess Rin and what he's waiting for.
But there's one major flaw with the second interpretation. If Satan can't exist in Assiah, what forces Rin to listen to him? It can end up like Mephisto. This plan as so many chances to backfire, I wouldn't call that a master plan.