Like it says on the tin.
When the portals begin to open, there's a portal extra that Strange didn't call for. Out of that one portal comes the former God of War, Kratos and his sidekick, Boy(who's instructed to go fight the rest of the army to gain experience, stay away from the heavy hitters, and leave Thanos for his father.)
And with that, the battle is joined!
GOW4 Kratos with his NewGame+ fully upgraded gear, VS Endgame final battle Thanos.
In the middle of the final battle.
Can Kratos turn the tide in a way that Tony doesn't have to make the sacrifice?