I guess it's not as clear as it once was. It all depends on how you interpret the flying rules.
"Fly Speed: To fly, a creature takes the walk, run, or charge action but uses its fly speed in place of its walking speed..."
You can clearly walk, run, and charge while flying.
"A creature that has a fly speed can also shift..."
And you can clearly shift
"...and take other move actions, as appropriate, while flying."
This last part is where it gets muddy. Crawl is clearly a move action, but what does "as appropriate" really mean? It could mean that you can use you fly speed to crawl while prone, since crawl is an appropriate move action while prone. But being prone doesn't just mean lying face down on the ground (as opposed to supine which is on the ground, but face up). It's also a state of being disoriented which is why you fall while climbing and flying and why you can knock prone creatures that lack legs or a clear body orientation.
Thus I tend to rule that you have to take a move action to "get your bearings" stand before you can move normally.
But I suppose what that really means is that being prone limits you to the crawl or stand move actions, but doesn't at all answer the question of whether you can use your fly speed to crawl.
"Falling Prone: If a creature is knocked prone while it is flying, it falls. This means a flying creature falls when it becomes unconscious or is subject to any other effect that knocks it prone. The creature isn’t actually prone until it lands and takes falling damage."
Again the rule says that you fall if knocked prone while flying, not are prone.
That being said, the power itself might not allow you to move while prone.
"Before the attack, you can fly 6 squares and must land in a square adjacent to your oath of enmity target. This movement does not provoke opportunity attacks."
The problem is that fly is not a defined game term. Flying and fly speed are defined, but not fly. If I were to reword this power I would word it as "Before the attack you can take a free action to shift 6 squares as though you had a fly speed. You must end this move on a surface that can support you and in a square adjacent to your oath of enmity target."
Also, land is not well defined. Can you land in a water square? Do you have to make an Atheltics check?
So I guess you'll have to ask [MENTION=75065]jbear[/MENTION] how this should work.