Main mission - Ebb and Flow | Act 4 Dungeon Siege III Guide

After you complete Enemy at the Gates and Uninvited Guests, the Queen (M29,1) will ask you to escort Phineas to the grand crystal in the middle of the underground (M29,10). Once there, protect the scientist from enemies until the Stonestream is turned off and afterwards return to aid the Queen in battle.

The enemies respawn every couple seconds - fight them off until Phineas finishes his spell.

Once you complete this mission, you will automatically receive Lord Devonsey's Trap.

M29 - Deep Well