Long Story short, Helm of Obedience targeting a player will exile your opponents deck if you control Rest in Peace or Leyline of the Void.

Helm of Obedience
Artifact
Card Text:
X, Tap: Target opponent puts cards from the top of his or her library into his or her graveyard until a creature card or X cards are put into that graveyard this way, whichever comes first. If a creature card is put into that graveyard this way, sacrifice Helm of Obedience and put that card onto the battlefield under your control. X can't be 0.


And


Rest in Peace
Enchantment

When Rest in Peace enters the battlefield, exile all cards from all graveyards.
If a card or token would be put into a graveyard from anywhere, exile it instead.


Your library consists of one card.

Blightsteel Colossus

Artifact Creature — Golem

Trample, infect, indestructible
If Blightsteel Colossus would be put into a graveyard from anywhere, reveal Blightsteel Colossus and shuffle it into its owner's library instead.

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Correct me when I'm incorrect: When you are targeted by Helm of Obedience, and you ‘mill’ Blightsteel Colossus, you can choose which replacement effect to apply - the one from Rest in Peace or Blightsteel Colossus. One causes the Blightsteel to be shuffled into the library; the other exiled. Choosing the former will cause the Blightsteel to get milled again. This is a loop - and the owner of the Blightsteel Colossus must inevitably choose to exile the Blightsteel Colossus to Rest in Peace. When a loop is established, a player does not have to choose to perform any action that would end the loop other than effects from cards involved in said loop.

Yep, that is correct. The affected player has two choices if the Colossus would go to the graveyard: either to shuffle it back in to the library to the Colossus' replacement effect, or to exile it because of the Leyline/Peace. Even if they keep choosing to shuffle it back in, at some point, they're going to have to apply the other replacement effect and exile the Colossuus. This is covered by rule 716.3:

716.3. Sometimes a loop can be fragmented, meaning that each player involved in the loop performs an independent action that results in the same game state being reached multiple times. If that happens, the active player (or, if the active player is not involved in the loop, the first player in turn order who is involved) must then make a different game choice so the loop does not continue.

Nathan Long
DCI Level 2 Judge
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