Idruun is a sphinx planeswalker. I want the library manipulation ability to be more of a "game," but there's simply not enough room.

Yow. Scary with Magmaw or Claws of Gix. But it's a 6-mana mythic and it does need external help to get truly scary, so... if anything it's probably too weak!

The last ability, Emblem. emblems arn't unique right? So after like 3 of these it's probably instant win.

Well, yes.

But he's no worse used just for that than a creature that grows +1/+1 every time it hits you; dealing 1, then 2, then3... - such creatures exist, and are potent. This is unblockable - just pure lifelosss (and can go 1/2/4/4 or 1/2/3/6/6 etc if it likes); but then, unlike a creature, this can be directly attacked. So that kinda cancels out.

And unlike the potent growing creatures, this is a six mana gold mythic. The main ability - "Don't kill me, I let you cast other peoples stuff, no matter HOW it got exiled" is the real interestingly mythic powerful bit.
To the extent that opponents are gonna look for ways to give this extra life if they can't remove it right away :)

I'm thinking his bottom ability should be -2, because six of those emblems is a lot.

Maybe, but then the top ability goes off a LOT sooner and that's very nasty.

Six of those emblems means it's at least turn 12 AND the opponent didn't do ANYTHING to this planeswalker in the meantime. It's a lotta damage, though, for sure.

Maybe some way of getting rid of the emblems when this leaves play? But the card's already very wordy.

Well, the fun tension in making the first ability go off sooner is that you have to have exiled opponents' cards first. And sure, you can do that with Oblivion Rings and things, but if you want to use this card's built-in way to exile cards, you're taking time off from ticking down to the ultimate. So it's at least 4 turns to get any free spells with this. (Admittedly you've also acquired three Underworld Dreams in the meantime.)

So I support changing it to -2.

Oooh, I hadn't thought of an ring.

So you case of the ring, use this to cast the creature you exiled and then the ring gets removed... what happens next? I guess it doesn't come back, because the ring lost memory of it?

What if "Exile until" wording was used?

In a similar manner to using Pull from Eternity on cards with Imprint or things like Oblivion Ring, Idruun's first ability would remove all relation between the two objects. Oblivion Ring would remain on the battlefield, unaffected.
One thing to consider about Idruun is that, as powerful as his abilities might look, he probably won't see any play by the competitive crowd with this set since he does nothing to affect the board and doesn't protect himself.

Oh, absolutely. That was what I was alluding to with my "if anything he's too weak" earlier.

An occasional mythic aimed at Johnny (with a splash of Timmy-appeal) is fine, of course.

I think griefer-Timmy would love this card.