If you defy a card, does it still ready Darius?

If you use Defy to counter a card, that card is not considered to have been played. Consequently, it will not trigger any abilities that rely on a card being played, including the ability of Darius, Trifarian to ready himself.

According to Rule 425.1.b:

"A card that is Countered is not considered to have been played."

Furthermore, Rule 419.4.b states:

"If a game effect prevents the resolution of the card—for example, because the card was countered—the card wasn't played and no abilities that trigger on playing cards trigger."

Sequence of events:

1. You play a card (the first card of the turn).
2. You attempt to play a second card.
3. Your opponent plays Defy in response to that second card.
4. Defy resolves and counters the second card.
5. Because the second card was countered, it is not considered "played."
6. Since the condition "When you play your second card" was never fulfilled, Darius, Trifarian does not trigger and will not ready himself.