A German rapper whom stardom eluded has achieved twisted infamy with ISIS, appearing on videos gleefully hoisting a severed head and exhorting other radical Muslims to leave the Fatherland and join him on the killing fields of Syria and Iraq.

Deso Dogg, whose real name is Denis Cuspert, traded a middling career on the Berlin music scene to become the face of German-grown terror, first in Al Qaeda, and then in the Islamic State’s so-called caliphate. In recent years, Cuspert, 39, who the U.S. State Department on Monday designated a “global terrorist,” appeared in numerous propaganda videos aimed at recruiting German jihadists. But he was rumored to have been killed until he appeared in an especially grisly video that surfaced in November.

"Cuspert is the most well-known of Islamic State German fighters.”

- German terror expert

In the video, which was made public by a Raqqa-based activist group, Cuspert is seen with other fighters who are shown shooting one person and beheading another. Cuspert is not shown killing anyone, but  holds the severed head and announces that the dead were enemies of ISIS.

“That's why they've received the death sentence,” Cuspert announces in German on the video.

Although the U.S. action came this week, Germany has long known of Cuspert’s bloody exploits in Syria.

"Denis Cuspert stands in the focus of security circles because of his essential role for Islamic State,” a German law enforcement official told FoxNews.com. “He is propagandist of IS."

Another Berlin-based source who has taken part in investigations of Cuspert, told FoxNews.com the onetime rapper, who now identifies himself as Abu Talha al-Almani, and Austrian cleric Mohammed Mahmoud are the leaders of Islamic State’s German-speaking contingent.

"Cuspert is the most well-known of Islamic State German fighters,” said a German terrorism expert who has tracked Cuspert. “If he is not killed, he will remain a long time [as a member of] the Islamic State. There are [numerous] reports of him present at executions involving decapitations.

The U.S. decision to designate a European citizen as a "global terrorist" is rare, and came as German Chancellor Angela Merkel was visiting Washington. The action bans Americans from doing business with Cuspert and freezes any of his U.S. assets. But it is more likely a formality, as Cuspert is unlikely harboring plans to leave the Islamic State’s so-called caliphate alive.

Well before Cuspert became a killer for the cause of radical Islam, he was a petty criminal whose rap lyrics revealed a dark and twisted mind. The son of a Ghanian father who left Cuspert’s German mother, he recorded three albums for a Berlin-based gangsta rap label, toured with American rapper DMX and scored a minor hit with "Willkommen in meiner Welt" (Welcome to my World) in 2010.

“Welcome to my world full of hate and blood,” went part of the song. “Children’s souls weep softly when the black angels sing.”

But his career was interspersed with short jail stints and squabbles with promoters. According to a report in Vice.com, he converted to Islam in 2010 following a near-fatal car accident. It was then that his music began advocating violent jihad.

In 2011, Berlin prosecutors charged him with illegal possession of weapons after Cuspert appeared brandishing weapons as "Abou Maleeq" in a YouTube video. A police raid on his home yielded weapons and ammunition, and although it did not result in jail time, he was squarely on the radar of German counter-terrorism investigators.

In 2012, Cuspert left Germany for Egypt, before eventually making his way to Syria where he joined Al Qaeda. When Islamic State broke away from the terror group behind 9/11, Cuspert pledged his loyalty to ISIS leader Abu Bakr al-Baghdadi.

In September, 2014, Berlin’s domestic intelligence agency—Verfassungsschutz—published a 25-page analysis of Cuspert. Entitled “Denis Cuspert—a Jihadi career,” the report said he “embodied one of the first Salafist propagandists as a committed and determined Jihadist” to emerge in German society.

A study by King's College in London last year estimated that 3,000 foreigners from Western nations are actively involved in ISIS, with 320 Germans among them. Foreign fighters have taken part in some of the terrorist group’s most barbaric crimes, including the massacres of the Sh’aitat tribe in Syria and the Albu Nimr tribe in Iraq, as well as the almost daily public executions in Raqqa.

Benjamin Weinthal reports on human rights in the Middle East and is a fellow at the Foundation for Defense of Democracies. Follow him on Twitter @BenWeinthal