I am 99% certain I know the answer to this but it's a piece of information I would like to have for my own reference.

I control crazed goblin and forcefield. my opponent controls meglonoth. I attack with Goblin, they block. can I use my forcefield to prevent some of the damage? Meglonoth is not a blocked creature, so logically speaking it is an unblocked creature and thus a valid target for forcefield.

First, the CR glossary for "Unblocked Creature" defines it as
"
Unblocked Creature
An attacking creature once no creature has been declared as a blocker for it, unless an effect has caused it to become blocked. It remains an unblocked creature until it’s removed from combat or the combat phase ends, whichever comes first. See rule 509, “Declare Blockers Step.”
"

So only an attacking creature can be an unblocked creature.


But even if the Forcefield didn't have the 'unblocked creature' requirement, this still wouldn't do what you want it to do. The damage Meglonoth deals with its triggered ability isn't combat damage. Forcefield ("1: The next time an unblocked creature of your choice would deal combat damage to you this turn, prevent all but 1 of that damage.") only prevents combat damage.

yeah it was pretty clear once you read the Oracle text on forcefield.

that was the answer I was actually looking for, I just had to try to construct a scenario where the distinction could matter. looks like I screwed up reading Meglonoth, I glanced at it and misremembered it as a replacement effect, which would as far as I know still count as combat damage. but yeah it's a trigger so doesn't really matter.

Looking for a creature that, as a blocking creature, could deal its combat damage to a player? Well, your own Goblin Psychopath can (to you).

ah, there you go. and, in that case, you can't prevent it with forcefield, yes?

basically I'd like to make the case that "whenever ~ becomes unblocked" is a better trigger event for things like Master of Cruelties and before I got into arguing clarity I wanted to first make sure that it didn't have any rules complications. but it looks like the rules do indeed support my assertion that a creature can only be unblocked if it is a) attacking and b) blockers have been declared.

I believe if you attack with Goblin Psychopath, coinflip decides it's going to be dealing its combat damage to you, and then it is not blocked, you can Forcefield it and keep it from hurting you as much.

If your Goblin Psychopath is being declared as a blocker and you lose the coin flip, it will deal its combat damage to you, but Forcefield isn't usable because it's not an unblocked creature (it is neither a blocked nor unblocked creature - those are states only defined for post-blocker-declaration attacking creatures).

It is still combat damage though. You could Fog it. You could prevent some or all of it with activations of Decorated Griffin.

You can also block an attacking creature with Butcher Orgg and choose to deal the Orgg's combat damage to yourself, and if so you won't be able to prevent the damage with Forcefield. Sounds like a great idea

Oh, yeah, that reminds me--I kind of want to see Butcher Orgg's little loophole closed, but it's so minor I can't really justify trying to get the rules team to waste time on it.