The former IWGP Heavyweight Champion entered the WWE World Heavyweight Championship Royal Rumble match at #3, after #1 entrant and defending champion Roman Reigns eliminated #2 entrant Rusev.
This isn't the first time Styles set foot in a WWE ring. He had his first WWE (then WWF) match against The Hurricane in WWF Metal in January 26, 2002. When a WWE contract didn't materialize for him, he went to TNA and became its NWA and TNA World Heavyweight Champion on separate occasions. Then, he joined New Japan Pro Wrestling, became the leader of arguably the greatest wrestling faction today, the Bullet Club, and won the IWGP Heavyweight Championship.
Welcome "back" to the WWE, AJ Styles!