I killed the Dark Elf and took the Skull of Corruption. I didn't ruin this achievement somehow did I?

no....


not killing him might have though.

Do I have to have all 15 at once to get the achivement? I had planned on acquiring them all and storing them in my home. I'd much rather sell some but yeah.

as long as you have had them in your inventory at some point it counts towards the achievement

Make sure to take the Masque of Clavicus Vile , not the Rueful Axe . The first is an artifact the other is not .
Be sure to do exactly what Mehrunes Dagon asks you to do without arguing if you want the Razor .
You lucked out on the Skull because if you hadn't killed Eramur , then no daedric artifact , and no achievement for you .
If you somehow mess one up , I've heard it is possible to get both Hircines Ring and the Saviors Hide as rewards for doing ILL Met by Moonlight . You'll have to google it though , not sure how to do it .

You don't need them all in your possession at once for the achievement according to the guide , but to be safe I would hold onto them . It's a long process getting them all , and sometimes there are glitches with the Daedric questline .

Good luck !

Oh snap.... I did the Hircine quest and got the Savior hide and he took the ring. Too late to go back and try to get both. Hopefully his ring isn't apart of the requirement?

Is the Skeleton Key counted towards it? I wasn't intending on doing the Thieves Guild this time around and want to get the the achievement...

No , it's either or . Getting both is just a failsafe if you muck up one of the 3 that you can miss out on .
The 3 you can mess up are the Skull of Corruption , Mehrunes Razor , and the Masque of Clavicus Vile ( he tries to trick you out of it, the first 2 can be missed by not doing what the daedra wishes you to do ) .

Skeleton Key is a Daedric artifact , but does NOT count , even though the guide says it does .
I know this from personal experience .

Thanks It means I don't have to become a sneaky thief !

No problem .
To clarify my statement , there are 17 Daedric artifacts in the game counting the Skeleton Key . Only the other 16 count towards the achievement , so even if you get the Key you still have to get 15 of the other 16 for the achievement . You normally can only get 15 of the other 16 artifacts , but you can miss out on up to 3 of them . There is a workaround on the Hircine quest that allows you to claim both of the artifacts associated with that quest instead of 1 or the other , thus allowing you to miss out on one ( and only 1 ) of the 3 you can mess up , and still get the achievement .

You can still get it. I got the achievement, but Hircine took the ring as well.

I actually got the achievemnt ( xbox 360 ) being two off the 15 I got it when I got both rewards from Hircine.