Terisian Mindbreaker

Activating a card's unearth ability isn't the same as casting that card. The unearth ability is put on the stack, but the card is not. Spells and abilities that interact with activated abilities (such as Defabricate's second mode) will interact with unearth, but spells and abilities that interact with spells (such as Scatter Ray) will not.

If you attack the same player with two Terisian Mindbreakers, that player will mill half their library as the first Mindbreaker's ability resolves. Then they will mill half of what remains in their library as the second Mindbreaker's ability resolves. They do not mill their entire library unless their library only had three or fewer cards to begin with.

If a permanent returned to the battlefield with unearth would leave the battlefield for any reason, it's exiled instead—unless the spell or ability that's causing the permanent to leave the battlefield is actually trying to exile it! In that case, it succeeds at exiling it. If that spell or ability later returns the card to the battlefield (as Static Net might, for example), the permanent card will return to the battlefield as a new object with no relation to its previous existence. The unearth effects will no longer apply to it.