En route to finding Branka during A Paragon of Her Kind, the party will pass through Ortan Thaig. Deep in the tunnels, Ruck will appear and scurry away leaving the party to deal with a surprise attack of some spiders. After, you can follow him to his cave. When you speak to the dwarf, you will learn he has become tainted from eating the flesh of darkspawn and his mind has deteriorated. You have a couple of options at this point:
Kill him;
Intimidate or persuade him to talk with you, which offers an opportunity to trade with him and two additional options:
Tell him you will let his mother know where he is;
Tell him you will tell his mother he is dead; this is his preference because he is ashamed of what he has become. Wynne approves (+2)
If Zevran is in your party when you talk to Ruck, when you choose the conversation option "I should go," he will advocate a mercy killing, saying it pains him to see Ruck like this. You will have no choice but to agree with him and attack Ruck. Either don't take Zevran along or ask Ruck to trade and exit the conversation with him by closing the trade window if you want to let Ruck live. Oghren will also suggest you put Ruck out of his misery, but he'll leave the decision up to you.
Return to Filda, who offers two more choices if you let Ruck live (regardless of your promise to Ruck):
(Lie) Tell her her son died. Wynne approves (+1)
Tell her he lived, but has been tainted by darkspawn and is a little less than sane from his isolation from society.
And two other choices if you killed him:
(Lie) Tell her you couldn't find anything.
Tell her he'd gone mad and you put him out of his misery.
There is an option that comes up if you talked with Ruck and killed him:
I'm not sure you want to hear it
(Lie) I found his remains. He died heroically, long ago. Wynne approves (+1)
Choosing this option will grant you the gold, experience and Ruck's Shield.
If Ruck lived, and you tell Filda that he died, she will offer her thanks, 100 Exp and Ruck's Shield (confirmed for PC and v 1.0.4). If you killed him and lie about it, she'll thank you for looking anyway and you'll get experience but not the gold or shield. If you killed him and tell her so, she'll curse you and again all you get is experience.
Alternatively, instead of searching for Ruck, or after you found him, you can lie to Filda and tell her you looked, but could not find him. You will get some gold, but not the shield. Sten, if in the party, will chastise you for lying to her. Should you tell Filda he's speaking rubbish, Sten will tell her Ruck's fate. No approval change. If you wish to lie, remove Sten from the active party.
If you choose the alternate route of lying to Filda before you enter the tunnels, you will not be able to change your mind later. However, Filda will still appear with a marker over her head, as if the quest has not been resolved.
If you talk to Ruck after completing the quest, Filda will appear with quest marker over her head, but will not speak with you.
If you tell Filda that Ruck is alive, she will say that she plans to look for him. However, she seems to remain in her same spot.
If you talk to Ruck before talking to Filda, then Filda is nowhere to be found, making the quest impossible to complete.