Sasuke split off a portion of his mental energy and used the Spirit Transformation Technique to enter the Kamui Dimension. The instant he discovered Obito there, he could not stop the corners of his mouth from rising.
With a thought, he activated Flying Thunder God. Leaving a Shadow Clone behind to coordinate with Naruto, he sent his real body straight into the Kamui Dimension.
Now that he had confirmed Obito could not actively sever the connection between the Kamui Dimension and the outside world, he grew bolder.
In the past, he had not dared venture casually into that mysterious dimension. The possibility was slim, but what if Obito got desperate, and Nagato somehow swallowed his bullshit and brought the entire Akatsuki into the Kamui Dimension to ambush him? He would be utterly screwed.
He might step inside with Flying Thunder God, ready to give Obito a vicious stab in the ass and teach him a lesson-show him that last night's booze was not the only thing that could bring tears to his eyes.
Only to find himself surrounded by a dozen burly men the moment he arrived. Once the Kamui Dimension closed, cutting off all contact with the outside, they could shut the door and beat the dog, catch the turtle in the jar.
He would get a direct lesson in pain.
Caught unprepared by a carefully laid ambush, even Sasuke was not arrogant enough to believe he would be in no danger. The Akatsuki was the ultimate team-fighting dream team. Together, those people were a roaring blaze, practically invincible in a group battle. Even if his arrival as the ultimate shit-stirrer had cost them one of their biggest heavy hitters, Uchiha Itachi, that did nothing to diminish how powerful they were.
The odds of Nagato trusting Obito Uchiha enough