In Search of Amgarrak

Acquisition
Given to you at the start of the Golems of Amgarrak DLC.

Walkthrough
You will start off at the start of the Deep Roads Grotto in the company of Jerrik Dace and his bronto, Snug. After finishing the conversation with him, follow the passageway. You will pass two parties of dead dwarves scattered around triggered traps (the second of which is lootable) and will eventually run into a group of darkspawn. After dispatching them, continue down the passageway to end up in a misty chamber. After killing a group of shrieks, the mist will shift.

Enter the newly opened chamber to encounter the Runic Golem and recieve The Runic Golem quest. After activating the golem, enter the outskirts of the Thaig by following the now-opened south-easternmost tunnel. After witnessing several Fade shifts (which are the work of the Amgarrak Lyrium Wells, encountered later) follow the resulting blue spectre until you will encounter an Elite Boss-level Arcane Horror, as well as an assortment of Undead. Jerrik will remark that they are the cause of the strange goings-on.

Finally, the spectre will lead you to the doors of Amgarrak. Jerrik confirms this, and you are now free to enter the thaig.

Result
The Warden gains access to Amgarrak.