The battle scene is one of the more uniquely choreographed fight scenes in Game of Thrones, but because the framerate is so incredibly fast, it’s hard to pay attention to what’s happening in the background. If you slow it down, however, and really pay attention, you’ll notice his crew cutting out the tongues of those they have captured. This may seem like nothing more than a deranged torture method, but there’s actually a reason for the slicing.
In the books, Euron Greyjoy would cut the tongues out of the men whose lives he chose to spare after raiding their ships. The idea was to essentially not allow them to talk back, question his command or enact mutiny against him during their travels. Greyjoy would collect these men and, in return for sparing their lives, they would help him during his raids, cutting out the tongues of men they captured and keeping the vicious cycle going.