It is general consensus that Illidan is currently one of the weakest characters in the game, outdamaged by the other assassins, outsustained by most heroes, basically what Illidan can do, other heroes can do it better. But just because he's bottom tier doesn't mean he's not viable.

Lately we're seeing a new build for Illidan which focuses on sustainability with shadow shield, immolation etc, and while he's able to last longer in the middle of the fight, I feel like his damage is lacking. So I went back to the old build of building him super squishy but in return his single target damage is optimal, which works very well with his Heroic Ability "The Hunt". What's surprising is that unless you got cc'ed hard to the ground, this build actually keeps you alive longer than expected because of the insane lifesteal you'll get from the enhanced basic attacks.

That said, both builds are certainly viable, but I'll be focusing on the true ganking assassin build.

Edit: With the changes to Path of the Marksman, Illidan now has access to being a carry!

Early on, you're very vulnerable as your damage and damage are pretty low. So farm safely and only engage if the enemy is alone and out of position. With the changes to Seasoned Marksman, I now get it almost all the time to get that extra oomph in your damage output. The only time I get Battered Assault nowadays is on Blackheart's Bay where you really have little to no time staying in lane.

Your main objective here is to farm that marksman damage, so AVOID SKIRMISHES AS MUCH AS POSSIBLE. When you predict an upcoming teamfight, carefully decide whether the team needs you or not, and if they don't, stay in lane. Getting fed while not dying is of utmost importance early on.

Mid Game is where you start to shine. Your damage continues to ramp up starting from level 10 with The Hunt, and of course you'll be able to pick someone off really easy with your heroic's cast range. Be careful when doing so though, you might want run into his teammates and that almost guarantees a death on your part.

Map Awareness is the key in picking someone off perfectly. You need to know where the opponents are, as well as where your teammates are in case you need to escape.

However if you can still stay in lane and ramp up your marksman even more, keep doing so.

Note: I recommend blood for blood almost all the time at level 16, but if your team is winning significantly that you wouldn't even need the extra lifesteal and your team has tons of CC, by all means get Executioner. It'll ramp up your damage output significantly!

It's basically like the mid game, only you can pick someone off regardless of where you are on the map. The only exception to the rule is you cannot Hunt someone when you're inside the mines and your target is outside, and vice versa. Now your marksman stacks will come into play. Your basic attacks will deal 300+ damage which multiples even further with the talents you picked up for Q and W.

What I see most Illidans do wrong is gank properly, from rushing down without proper timing, being completely out of position, etc.

There are few things Illidan is better at than all the other heroes, and that is the ability to position yourself for a body block. When ganking with a teammate, use your Q to position yourself in front of your target's escape path, and proceed to body block while attack cancelling. This should give you at least a couple more seconds to kill the target.

Be careful when you use sweeping strike, as it has a longer cooldown than Dive. Carefully plan if you can kill your target before backup arrives, and if you can't, W away to give you some space. Illidan is a great 1v1 hero, but his kill potential drops when numbers are not in his favor, more significantly than any other hero in the game.

Also, do NOT use E immediately after you just dived in. You're not in an immediate threat when you're the one ganking, so save it either when you get counter ganked, or you're about to finish someone off in their tower.

Illidan is without a doubt Blizzard's most badass character, and how he is played in Heroes further cements that. He's so fun to play, and in the right hands, tough to kill. It will be pretty hard to learn when to engage or disengage, but once you are prepared (see what i did there? :D) let them feel the hatred of 10,000 years! :D