Hello I am looking for clarification on Sensing Eye also. i run a dark sun game and a player of mine uses it to gain combat advantage. Ill do my best to explain. According to the Rule of Combat Advantage "...The target is pressed by multiple enemies at the same time, stunned, distracted, or otherwise caught off guard. Combat advantage has two rules...Able to see the target. A creature must be able to see the target to gain combat advantage against it. S=Sensing Eye E=Enemy W=Gloom Pact Hex-Blade with reach A=Ally
The warlock uses the Eye to gain combat advantage because he is either flanking or catching the enemy off guard. I dont understand that power too well but that seems like it was not intended to be used that way. I think they are using it to catch the enemy off guard by seeing it from a different perspective. Your insight would be much appreciated.
The hex blade nor his ally would not get flanking in this scenario, because he is still attacking from his own square, and the sensing eye does not threaten. Nothing about that drawing you have above gives anybody combat advantage from flanking. Sensing eye only gives line of sight and nothing else. It doesn't even conjure an image of an eye in the square - it quite literally only allows the psion to determine line of sight from that square for the next turn.
You can only get flanking by being adjacent to a creature while one of your allies is adjacent and on the opposite side (a few feats let you be one square over, but those are specific exceptions to the rule).
Now, using the scenario you drew above, lets say there's a sandstorm or some other effect giving concealment to anyone more than 5 feet away. If the enemy were granting combat advantage for some reason other than flanking (like its Dazed), the hex-blade would not normally be able to get that combat advantage with reach or a ranged attack because the sandstorm grants concealment beyond 5 feet However, with the sensing eye adjacent to the target, he can now see it clearly and he would get to use the combat advantage from the target being dazed.