The Leech King is a Mage Champion based on some existing mechanics but he has some twists that make him a unique character and give him his very own feel. His theme is 'Strength in Numbers'.
The story behind the leach king is based on those of Tarzan and Mowgli, where a young child grows up in the wilderness getting tought survival skills by the local wild life. There are small twists however, the child is a bastard of Boram Darkwill the ruler of Noxus, who usually kills his offspring when they become a threat. And on top of that the animals are not exactly brimming with compassion for the young man either.
His Q aplies one leech to target foe, exploding when it dies dealing a small amount of damage and bonus damage for each second it was alive.
His W releases a macial shout that forcibly explodes all leeches in range. Leeches that exploded in this maner apply a short duration fear.
His E strengthens the natural bond between comrads, increasing his own and his allies life steal and spell vamp for a small amount for each ally in range (of the leech king, its a buff like Gangplanks Raise Moral)
His R opens a rift to the void, each second a foe stands within this rift a leech jumps out and latches on. Everytime a leech explodes over the rift it decreases the armor and magic resist of the targeted champion.
The goal for his skill set is to be able to sustain his team and still bring out some disruption and debuffs. He can burst one target by spamming his Q on it, or he can spread around his Q to apply a disrupting fear on an entire team. Seeing as it is a mass fear, it's duration will be short, probably one second max, one fith second per leech. His E is his main supporting skill but it's effectiveness depends on how much damage his allies are doing.
And his Ultimate combination is quite devistating as he can lay down the R, then rush forward to spam Q, heal himself a bit with E and keep the enemy on the ultimate with the fear from W triggering the armor and magic resist debuff.
His counter would be to disrupt him as all his damage comes from the abillity to link his skills together. If he initiates with his short duration R, stun him directly so he cannot activate W giving you time to step off the rift.