In short, our mandate was to claim revenge for our brothers and sisters who fell when they were betrayed by Oren, long ago. It has branched out over the eras, but do not take our hate of Oren a discouraging factor; we welcome all races and skill types.
Should mention, we no longer have much hate for Oren as a major factor in the guild. There's still a wee bit of bad blood due to past conflicts, but that's about all. Some of the guild's philosophy actually plays it up as more of a necessity to the world than an enemy.