So I'm playing Martyr on PS4, using Crusader class, and can't figure out how to do executions. I've checked the tutorial and it says a marker will appear above an enemy's head and then I have to press X within a limited time. What does the marker look like? I've also been investing points in the Execution skill tree, but am still at a loss as to what I'm meant to be doing. Anyone else having this problem?