If this character's attack hits, all characters adjacent to the target take 10 damage; save 11. If the attack misses, the target and all adjacent characters take 10 damage; save 11.
If this character’s attack hits, the target takes the attack’s damage as normal and all characters adjacent to the target take 10 points of damage from Splash (enemies and allies alike). If the attack misses, the target and all adjacent characters take only 10 points of Splash damage. In either case, a character can avoid the Splash damage with a save of 11. Special abilities and Force powers that redirect damage don't work against Splash (although if the original attack hits, the target can attempt to redirect that attack's damage).