Kaiser debuted for WWE's developmental territory NXT in June 2017 under the name Marcel Barthel, losing to Roderick Strong at a live event. In August 2018, Marcel Barthel made his NXT television debut (taped in July) as a heel in a loss against Keith Lee. Marcel Barthel was defeated by EC3 on the November 28, 2018 NXT. In December and January, Barthel then formed a tag team with Fabian Aichner. The duo, dubbed the "European Union", then went on to score wins on both NXT and NXT UK. While in NXT UK, Barthel and Aichner united with Walter (and later Alexander Wolfe) to form the stable known as Imperium. On May 13, 2020, Barthel and Aichner defeated Matt Riddle and Timothy Thatcher (a substitute for Pete Dunne) to win the NXT Tag Team Championship. They would go on to successfully defend the titles against teams such as Oney Lorcan and Danny Burch, Breezango, and The Undisputed Era. On the August 26 episode of NXT, Imperium dropped the titles to Breezango, ending their reign at 105 days. They failed to regain the titles from them in a rematch on the September 16 episode of NXT.
Barthel and Aichner would later defeat MSK in a lumberjack match on the October 26, 2021 Halloween Havoc event to win their second NXT Tag Team Championship. They would successfully defend their titles on the December 5 WarGames event, against Kyle O'Reilly and Von Wagner. On April 2, 2022, at NXT Stand and Deliver, they would lose the titles back to MSK in a triple threat tag team match also involving The Creed Brothers, ending their reign at 158 days.
On the April 8 episode of SmackDown, Barthel, under the new ring name Ludwig Kaiser, debuted with Gunther, accompanying him in his match against a local competitor. On the May 27 episode of SmackDown, Kaiser had his first match on the main roster, teaming with Gunther to defeat Drew Gulak and Intercontinental Champion Ricochet. A feud with the Brawling Brutes ensued for several months afterwards, pitting Kaiser, Gunther, and the debuting Giovanni Vinci (formerly Fabian Aichner) as a reformed Imperium. As part of the 2023 WWE Draft, Kaiser and his Imperium teammates Gunther and Vinci were drafted to the Raw brand. On the April 23, 2024 episode of Raw, Kaiser would attack Vinci after losing to The New Day (Kofi Kingston and Xavier Woods).
On the August 30 episode of SmackDown in his home country of Germany, Kaiser answered LA Knight's open challenge for the United States Championship, but failed to win the title.