Does Dauthi Voidwalker exile cards milled by your opponent as well as discarded cards?

Dauthi Voidwalker reads: 'If a card would be put into an opponent's graveyard from anywhere, exile it instead.' This replacement effect applies to any card moving to an opponent's graveyard, regardless of the source or cause — milling (putting cards from library to graveyard), discarding, dying in combat, being countered (for permanents), and so on.

Under CR 614.1, replacement effects modify what would happen to an event. Since the card would normally enter the graveyard, Dauthi Voidwalker's effect intercepts that event and redirects the card to exile instead. The method by which the card arrived at the graveyard zone is irrelevant — only the destination matters.

Discarding a card is specifically defined in CR 701.8 as putting that card into its owner's graveyard. Because that destination is an opponent's graveyard (from your perspective), Voidwalker's effect applies. Similarly, milling is defined in CR 701.13 as putting the top card(s) of a library into that player's graveyard, so that also triggers the exile replacement.

Example: Your opponent activates Hedron Crab's landfall trigger, milling three of your cards. Instead of going to your graveyard, all three cards are exiled face-down under Dauthi Voidwalker, and you can later sacrifice it to cast any one of them for free.

Note that this only applies to opponents' graveyards, not your own. Cards going to your own graveyard are completely unaffected by Dauthi Voidwalker's static ability.