You're able to affect a single square and 'trap' it with your reach.
EXP Cost: 4 EXP
Benefit(s): As a standard action you can strike a square within your melee reach and you're able to designate it as the origin of your 'Viper Fang'. Only one square at a time can be designated as one. This square is counted as part of your melee reach for the purpose of delivering attacks of opportunity, even if you move away from the square so long as it is within 18 squares of you. This ability lasts for 6 rounds.
Example: If Derek the Soldier creates a Viper Fang in the square directly north of him, and then takes a move action to move 6 squares back. He is counted as being in his square, but also the square of the Viper Fang for the purposes of delivering attacks of opportunity, even though his melee reach wouldn't normally extend that far.