When you enter the Mother's Nest, she speaks with you. No matter what you say, expect to end this adventure with an epic battle. It will test your combat skills and party tactics to the fullest. Seasoned Wardens can slay the Mother; others won't last 40 seconds.
At the start of the battle, expect to get hit hard. The Mother's giant tentacles erupt all around you: one on either side of the party and two in front of the Mother. The tentacles will begin slamming party members, or grab one and start crushing the unfortunate victim. The healer should be on alert to heal any party member in need, not just the tank, as the attacks can come from any side. If the healer gets grabbed, it's a big plus if you have a second healer, or at least a mage who can Force Field the healer. Failing that, you'll have to rely on health poultices.
Spread your party out to avoid AoE attacks. You don't want to get slammed by a concentration of giant tentacles, or even get hit by your own AoE damage. Also, keep in mind that it's easier to kill the Mother's tentacles than the Mother herself; however, each time a giant tentacle is slain, the Mother calls in a wave of childer grubs to swarm you. Wiping out tentacles is a good thing; wiping out multiple tentacles in a row is maybe not so good, as you'll spawn a lot of grubs to control at once.
Concentrate all your fire on a single giant tentacle and bring the second tentacle down. Don't worry about the Mother at all; you'll deal with her later, after you have the tentacles and grubs under control. The Mother doesn't move, so you know where her attacks are coming from at all times.
You'll hew down the third and fourth tentacles much quicker; they've already taken massive damage by the mid-point of the fight. You may even knock both of them out at once, which means you'll have twice as many grubs to control. Save some AoE to crush the grubs before they flank and overwhelm.
Once the fourth giant tentacle flops to the ground, engage the Mother. Melee DPSers need to get in tight to deal max damage, and the Mother will punish them with tentacle slaps or grabs. The healer may not be able to keep up, so melee party members should pop a health poultice whenever they drop near 50 percent.
Pour on all the damage you can to kill the Mother before she kills you. At this point, it doesn't matter if an ally drops in combat; you need to out-race the Mother in damage. If you have any Tevinter powers left, trigger them in these final seconds.
With the final blow, the creature that caused so much tragedy across the land of Amaranthine comes to an end. You slay the Mother so that she can no longer breed nightmarish children to plague the land. You walk away to repair an arling that needs more devotion to its people than to the art of warfare.