Celestial flare

[ibtrickey]

If a creature is attacking and a player plays this after blockers are declared, what happens?

[FustyDavorite]

 {Celestial Flare} you know the drill bud! Give me a second

[FustyDavorite]

Is this as in your opponents declares attackers, you declare blockers? I'm sorry the question seems vague.

[FustyDavorite]

I'll just set up a scenario. Your opponent attacks with a 3/3 flyer and a 2/1 vanilla. You block the 2/1 with a 1/1. In response to ending declare blockers step you cast  {Celestial Flare}, putting it on the stack. He sacs the 2/1, you proceed to combat damage step, you take 3 damage from the flyer, and the 1/1 has  an invalid target and does not deal damage.

[Giggle the Draco Genius]

I'll just set up a scenario. Your opponent attacks with a 3/3 flyer and a 2/1 vanilla. You block the 2/1 with a 1/1. In response to ending declare blockers step you cast  {Celestial Flare}, putting it on the stack. He sacs the 2/1, you proceed to combat damage step, you take 3 damage from the flyer, and the 1/1 has  an invalid target and does not deal damage.

That is correct, although I find casting the flare before declaring your blockers allows you to choose your blockers more effectively

[Pleeb]

I'll just set up a scenario. Your opponent attacks with a 3/3 flyer and a 2/1 vanilla. You block the 2/1 with a 1/1. In response to ending declare blockers step you cast  {Celestial Flare}, putting it on the stack. He sacs the 2/1, you proceed to combat damage step, you take 3 damage from the flyer, and the 1/1 has  an invalid target and does not deal damage.

You're close. The 1/1 isn't targeting anything, so can't have an illegal target. He is still a blocking creature until end of combat and can be hit with spells and effects that target blocking creatures. You are correct in that he will take no damage because the creature he was blocking is no longer in combat.

[MuggyWuggy]

If a creature is attacking and a player plays this after blockers are declared, what happens?

***Most likely played "in response to blocks declared and before damage is assigned"

You must sacrifice a creature if you are the target player.

Resolved.

[ibtrickey]

They attack. I block. They cast this.

[MuggyWuggy]

You sacrifice a creature.

[ibtrickey]

Does it still constitute as blocking? And deal its damage?

[Spikepit]

They attack.
You block.
Cast the flare.
You sac the blocker.
No damage dealt. It must a blocker to be sacced. The attacking creature does not keep charging at you as it has been intercepted (unless trample is involved).

A footy player charges down the field at full speed. A linebacker grabs him and they tussle. Suddenly, the line backpacker disappears! Would the player be instantly at full speed again?

[Pleeb]

Does it still constitute as blocking? And deal its damage?

{celestial flair}

After you declare your blocker, your opponent casts this targeting you. Damage hasn't been done yet. Flair resolves forcing you to sac your blocking creature. No other spells are cast and damage resolves.

Your blocking creature is no longer on the battlefield and so can't deal any damage. The creature he was blocking remains blocked, and so baring trample or another creature blocking it, will also deal no damage.