Just when you thought it was safe to enter an expansion jungle, think again. The Guild Wars 2 dev team is hard at work making Heart of Thorns as intimidating to the veteran adventurer as possible, and one of its tools is the fearsome wyvern.
The devs used the champion creature as an example of how the expansion’s improved defiance mechanic will work. The new system will show an additional bar under the boss’ health bar that represents its defiance (i.e., anti-crowd control protection). The more CC used on the creature, the more the bar degrades until the creature can be affected by such skills. However, the bar is always regenerating, so player groups will need to hit it fast and frequently to make a difference.
The new system can be used for interesting encounter, such as the wyvern. Players will attempt to stop the creature from taking off by depleting its defiance bar, and if they are successful, they will get a few seconds of free attack time. It may be worth it to let the creature attack you at least a few times, if only to see the wyvern’s “believable” fire effects.[Source: Guild Wars 2]