If I play the Harrowing and target darius Trifarian from my trash will he enter ready?

Yes — as long as The Harrowing is your first card played this turn.

The Harrowing resolving and playing Darius from your trash counts as playing a card (FAQ #9614). So the sequence is: The Harrowing = card 1, Darius entering = card 2. Since Darius is already on the board when the "second card" condition is checked (Rule 419.4.a; FAQ #1119), his ability fires — he gains +2 Might and readies himself, even though he entered exhausted like any unit played by an effect.

If you'd already played a card earlier that turn, The Harrowing would be your second card and Darius your third — past the trigger window — so he'd stay exhausted (FAQ #9420).