'Defeat the Shadowbringer and complete the 'Shadowbringer' quest'
Once you have found and defeated Brek, the Ork King, he will give you a quest to find and defeat five 'fallen'; a Shaman, a Berzerker, a Wolf Master, and Orc and a Chieftain.
Defeating all of those will allow you to break the seal in Brek's room. To break the seal, you simply need to gain 15 of each of the gems. This must be done within 25 turns. It's much easier than you might think, especially if you are good at connecting 4's and 5's, as well as using multipliers, etc.
Once you have broken the seal, Brek will tell you to defeat the Shadowbringer. He doesn't have too much health, but a few tricky spells. One of his spells converts many of the gems to purple. I used the spell 'Chasm' to take advantage of the purple gems, but it depends on what is available to you, and which strategies you are comfortable with using.
Chasm destroys the row and column of a selected purple gem, giving full effects, whilst also giving 6 damage for all purple gems destroyed.Please either comment below or send me a Personal Message if you have any comments or improvements.