It was Oberyn Martell all along!
The Mandalorian is arguably the greatest show ever made. We have Jon Favreau to thank for that! It stars a titular bounty hunter who lives by strict guidelines that prevent him from removing his helmet. Fans have seen countless other Mandalorians without their helmets on, but the show goes on to explain that.
Din Djarin, a.k.a. Mando, was brought up in a religious cult. His sect tore away from mainstream society, allowing him brief moments where he could remove his helmet. Fans were shocked to see Pedro Pascal under the mask, even though we’ve seen him a zillion times already in Game of Thrones.