I have all of the Deadric artifacts, but the trophy "Oblivion Walker" isn't popping. I even went back and killed Sinding, but Hircine wouldn't reappear.

What do I do to get the trophy to pop?

I never could get that trophy either. I even had all of the Daedric Artifacts too. A glitch maybe?

I hope not. It says "15" under the Deadric quests stat, and I have all of my artifacts- none have disappeared or anything.

Did you acquire all the artifacts that are actually counted as daedric artifacts?

http://www.uesp.net/wiki/Skyrim:Daedric_Quests

For example Rueful Axe is not counted as daedric artifact.

I have: Azura's Star, Dawnbreaker, Ebony Blade, Ebony Mail, Mace of Molag Bal, Masque of Clavicus Vile, Mehrune's Razor, Oghma Infinium, Ring of Hircine, Ring of Namira, Sanguine Rose, Skull of Corruption, Spellbreaker, Volendrung, Wabbajack

I should be good. I used the wikia guide to make sure I obtained all the right artifacts.

One thing you could try is to return to the Bloated Man's Grotto and kill Sinding if he's still there (apparently we should be able to see him running around Skyrim too, but I have never seen him outside of the cave). Getting his skin will increase your count by one more, which may trigger the achievement.

Thanks, but that was one of the first things I did. I went, killed him, took the hide, then left the grotto. Hircine, however, didn't reappear.

Oh, shoot. I'm afraid you're out of options then. :/ Are you on a PC? If yes you could try adding the hide via the console.

I gave up on that trophy a long time ago. I have the game on the PS3 console and some trophies I worked toward that I could not manage to achieve. Although I've heard this achievement is the hardest of them.

No, PS3. & yeah, that's what I'm starting to think; I'm S.O.L. for the time being. Thanks, anyway.

Dawnguard wiped the properties on DA03, which prevents the Masque of Clavicus Vile from counting for this achievement (if your save was started with Dawnguard installed, that is). Bethesda never fixed this issue, so the only way to get the achievement is to receive both reward for "Ill Met by Moonlight."

This is issue is fixed by the Unofficial Dawnguard Patch.

So, if I uninstall Dawnguard, then load the save right before I turned in the final Deadric quest (Peryite), will it solve the problem? Or, do I have to go back and do all the Deadric quests again without Dawnguard installed ("Ill Met by Moonlight" was one of the first ones I did)?

Is it possible to use one of my saves from when Dawnguard was installed after I uninstall it?

I'm afraid that won't work. It would only work if you'd reload the save from before doing the quest for Clavicus. In this save game file records are already set in place, removing Dawnguard will not go through the old records and fix what Dawnguard messed up. So yeah, all you can do, if you really want that achievement, is to remove Dawnguard, and do all daedric quests again, or keep dawnguard and get both artifacts from Ill Met By Moonlight, with a new character.

Ok, so I have a save (a few actually) that are before I did any of the Deadric quests. If I remove Dawnguard, I can still load one of those saves (that had Dawnguard installed at the time of the save), and it should all work out properly, as long as that save hasn't started any of the Deadric quests?

Sorry, I just want to make sure I understand before I go back through and do all the quests again.

You're good as long as you have a save before the messed up quest. in this case you can choose between removing Dawnguard and doing A Daedra's Best Friend, or keeping Dawnguard and doing Ill Met By Moonlight, and getting both the ring and the hide. Other quests don't matter, so if you have a save where you already did some daedric quests that aren't one of those two, it should be fine.

I tried going back to the grotto, and killing Sinding to obtain the hide. I got his hide, but Hircine wouldn't reappear.

That's really weird, it should happen in vanilla game. UESP doesn't mention any fixes of this glitch. Unless someone finds another way, uninstalling Dawnguard and doing Daedra's Best Friend again seems to be the only option left. :s