From Destinypedia, the Destiny wiki
Darkblades were first introduced in Destiny in the form of Alak-Hul, the Darkblade, a unique Strike boss fought at the end of The Sunless Cell. Weaker versions of Alak-Hul were carried over into Destiny 2 as mini-bosses, such as the Abyssal Champion. They are visually identical to Alak-Hul, except for a glowing line on their helmets similar to those of Destiny 2's Knights, and mechanically similar.
Darkblades will advance slowly towards the player and attempt to crush them with their axes. Like Alak-Hul, their helmets will break off when they take enough damage, causing them to become enraged and begin rushing at the player in a straight line. While they are easy to outmaneuver, one must be mindful of their surroundings to avoid being cornered while doing so.
Some Darkblades have access to a teleportation ability where they will dissipate into the air and reappear a few moments later, often behind and/or within striking distance of the player. One should stay on the move and watch out for the reappearance to keep their guard up.
List of Appearances