Three months later Nasser's younger brother Aseel also went out to join them.
I went to sixth form college with both Nasser and Reyaad, and Aseel had gone to my school.
I recently travelled to Syria for BBC Panorama to try and speak to Aseel and to find out why they left Cardiff to join such a brutal organisation.
Aseel was pleased to see me. He's spent two years in a crowded cell in north-east Syria. He is being held by the Kurdish forces who captured Islamic State group members at the last bloody battle of Baghuz.
He said he had been attracted to Syria by the revolution and civil war there. "People being killed and, you know, children dying, barrel bombs… The regime, what they were doing to the people."
Nearly 900 people from Britain went to join Islamic State. Many of them were recruited after watching online videos and reading messages appealing to a sense of empathy towards the suffering Syrian people.
I wanted to know what Aseel did when he arrived to alleviate the suffering of Syrians.
He couldn't answer me - because he had no answer.
Islamic State took over vast swathes of land in Syria and the movement spread to Iraq.
He described for me the day the so-called caliphate was declared. Now he was talking freely. His excitement was easy to feel; it was like he went back to that day and relived it.
He described the crowds cheering, beeping their horns as every kind of weapon fired off and lit up the sky. His hand even moved as he recalled waving the ISIS flag during the celebrations.
His smile was jarring. What he was describing was the takeover of Syrian and Iraqi towns and the civilians who lived there. The Islamic State group executed people who dissented or didn't comply with their policies.
When I told him this he replied: "I'll be honest, Islamic State, yeah, what they stood for was good. But, at the same time, I would say individuals… they did a lot of stuff that made the State look bad."
But it wasn't just a few random individuals. It is believed Nasser Muthana appears in a video of an ISIS execution by beheading; and he gloated that the UK should be afraid of his bomb-making skills if he ever came home.
Reyaad Khan was killed in a British drone strike. The government published a report which indicated that Reyaad Khan gave out bomb-making instructions and "identified targets" in the UK.
He was believed to be recruiting British Islamic State supporters to carry out attacks on high profile targets.
Speaking in 2015, then Prime Minster David Cameron said: "We should be under no illusion, their intention was the murder of British citizens, so on this occasion we ourselves took action."
Aseel described the day Reyaad was killed to me. He said: "My brother was devastated, he was crying, he was so angry. Reyaad died for that cause, you know?
"My brother was so adamant, [saying] 'I'm going to carry on his work'. Insisting. I told him, 'right you carry on your work, you do what you've got to do'."
Nasser Muthana did continue recruiting and speaking to Islamic State supporters in the UK.
Aseel said that Nasser had told him he was aiming for government officials or the army, "even if that means people dying".
We don't know if any of the operations he planned were successful, but he was killed in a targeted drone strike in 2017.
Nasser had been married once to a British woman and has one son - Almoughera.
Aseel married twice but he only has one surviving child with his first wife.
In trying to find Nasser and Aseel's wives, I spoke to dozens of women who are based in the camps.
Many of them are still pro-Islamic State group.
On their Instagram feeds you can see the black Islamic State flag, or an AK47 propped up against a fence. They boast about their children, calling them "cubs of the caliphate".
Aseel told me that Nasser wouldn't want his son to go back to Wales; he would have wanted him to be the future of ISIS.
I don't know what I expected when I went to meet Aseel. A small part of me thought he would say he really wanted to come home to Cardiff and he was sorry for what he'd done. I spoke to him for hours, but I didn't find that at all.
He shirked responsibility for joining a bloodthirsty cause and insisted his intentions had been good and that there was no evidence he had taken part in any violence. I left feeling deflated. There was a massive gulf between my world and his.
He is waiting in a prison until the international community decides what to do with foreign Islamic State fighters. He said he just wants the UK to make a decision on his fate, explaining: "It's not that I wanna go back, like, to UK, I just wanna be dealt with."
The whereabouts of his son and his nephew are unknown. The Islamic State group hasn't gone away and there are many ISIS cells still inside Syria.
If the group knocked down that prison wall tomorrow, I have no doubt he would go back to them.