I consider that Shadow Demon is a very misunderstood hero. His strength lies in the ability to open up weakness within the enemies armor with reletive safety.

You can deal out more DPS than a lot of middle heroes. The way S.D. delivers damage can be hard to deal with even by a good player. Either the enemy tries to wiggle around like a lunatic in order to dodge Shadow Poison, all while trying to last hit and deny ; or he leaves creep range in an attempt to let any stacks accumulated expire...and, even then, under tower isn't out of range of Shadow Poison. It's a lose-lose situation that can only be stopped by a lot of agression.

But, it's easy to overwhelm any opponent on any hero at level 3, just by right-clicking on them to get them around 60-80 percent. Then even getting ONE stack can be the clincher ; because then it's Disruption time, and another free stack. Then two illusions of the enemy popping out (they can trap people against the walls of river or in creeps), hitting him. You may even move up onto the enemy's high-ground in order to make escape difficult while they are Disruptioned. It's very easy to keep stacking poison on a person who is making a bee-line for their tower. Retreat makes pathing obvious, after all.

Once S.D. is level 6, ganking a piece of cake. Buy Smoke of Deceit or just TP to a lane. The slow from Demonic Purge and Disruption on any opponent within creep or ally range is a guarenteed kill in a lot of circumstance.

S.D. can be a powerful mid-game hero if positioned wisely. Just use Shadow Poison passive-agressively from maximum range. Try to use it to harrass people out of the lane, or force them to attack you. If they push forward to be agressive, back-off and see if any of them break's formation early, and if someone else doesn't stop chasing and follow them. They will put themselves out of position 99% of the time. This is when you use Disruption or Demonic Purge to overwhelm them and force their hand. Commit or die trying to run.

THE POWER IS IN YOUR HANDS! Be scheming and greedy and demonic! Your enemies will be crushed under your strength! CHAOS IS YOUR MINION!