Celtic Heroes

What do they grant? I don't understand what they do. Any ideas?

rogue dl offhand grants the dragonlord spite skill. It does not require energy to cast but only affects mobs and bosses vulnerable to mythic attacks. It works very much like rend and deals damage over 15 seconds. I noticed that in the 15 seconds, it hits 5 times (i.e. each tick is 3 sec) with damage ranging from 400 plus to 800 plus although the max damage is supposed to be as high as 1500 per tick. For one cast, the total damage over 15 seconds can range from 3k to 4k. See pic in next two posts.

Here is the description:

Skill description