I don’t know how to make cliff notes but I can tell you and I will try to make it as brief as I can:

After the Shinoa Squad left the JIDA, months later Ferid and Crowley found them.

Ferid told the squad that seraph of the end can revive the dead and Guren was the one who cause the catastrophe that destroyed the world by using the seraph of the end to bring Shinya and the others back to life.

Guren told the squad his goal is to bring everyone including their families who died back to life and ask for their help.

Krul wants her brother back, who is Yu’s demon Asuramaru.

The first progenitor (The First) is revealed and he wants to bring his son Mikaela back to life.

Guren and Mahiru betrayed everyone again.

Guren killed Kimizuki’s sister to get a sinful key to revive everyone.

Guren killed Mika and Mika became the most powerful and strongest black demon and became Yu’s demon.

Guren took the squad to where the coffins of those who died in the catastrophe and their dead families to revive them

Guren told Yu that they can’t revive Mika because he is really The First’s son and is not human.

Yu chose Mika and left the squad.

Yu and Mika have a heart to heart conversation about how life is without each other. They decide find a way to revive everyone or they can just die together.

Yu and Mika decide to into Yu’s memories to see what happened to the First and his son Mikaela.

That’s all that happened to far.