Flash, Evoke, and blocking

[Sparkle Ninja]

Ok, so I had a {prophet of Kruphix} on the field and a { Cloudthresher} in hand. The opponent attacks with a 12/12 without trample, can I cast {Cloudthresher} for its convoke cost in as a blocker and stop the 12/12?

[Remillo]

Short answer: No.

Long Answer:  Once your evoked Thresher resolves in the Declare Attackers step, two things will go on the stack - The Tresher's own ETB ability, as well as evoke's Sacrifice trigger.  Both of these end up resolving before you get to move to Declare Blockers, meaning your Tresher wont be on the board to be able to block (barring {Torpor Orb} or {Cloudshift} shenanigans)

[Munchlax]

Another example of evoke would be {Briarhorn}

[blackychan1]

I can cloudshift evoke creatures? And torpor orb stops the sacrifice? I must build a deck for this!

[particle]

I can cloudshift evoke creatures? And torpor orb stops the sacrifice? I must build a deck for this!

Playing a creature by paying its evoke cost will result in two comes-into-play abilities: The sacrifice ability from evoke, and whatever other ability the creature has. The creature's controller chooses in what order to put them on the stack. Both abilities can be responded to as normal