Shadow Demon/Guide

General
In most scenarios, Shadow Demon's combo goes like this: initiate on a lonesome enemy with ; use a to DISABLE HIS BLINK DAGGER should he have one, use  on top of it to amplify his damage taken, use  when he pops out of disruption, laugh maniacally as he dies from illusions of himself hitting him for an unexpected amount of damage, while stacking more poison on him, unless he has an easy way to purge the poison (dark pact, press the attack, repel...) in which case you might as well activate it on every 1-stack, and repeat the purge if you have a scepter.

Disruption, as most banishes, is a very versatile skill, that has this unfortunate flaw it can do more harm than good to your team than good, in the wrong hands.

Like eul's scepter of divinity, it is a perfect setup for unreliable stuns such as Leshrac's split earth,  Lina's Light-Strike-Array or  Kunkka's torrent. Like eul's, it is perfect to waste 2.5 seconds of an opponent's precious buff, such as Dazzle's shallow grave or  Abaddon's Borrowed time.

Unlike eul's, it can be used on allies to save them from opponents' initiation or even their finishing blow. Thus you can save an ally and turn a fight around by banishing him from a predictable Reaper's scythe or even a  Finger of Death or  Laguna blade. The same can be said for Outworld Devourer's Astral Imprisonment. In return, however, and for the same reasons, it can also serve as the perfect initiation tool for the enemy team!

Disruption also creates two very strong illusions of the target, making Shadow Demon very dangerous against illusion based heroes such as Terrorblade and  Phantom Lancer. If the target is an enemy, the illusions will automatically attack it. These same heroes will also work as your best partners when besieging the opponent's high ground at little to no risk. Special mention goes to Centaur warrunner whose illusions benefit from his return passive, damaging the towers for a considerable amount should they (automatically) target the illusions.

Plenty of heroes have an escape mechanism that can render your illusions useless after the disruption (blink, leap, pounce...), if no ally on your team has any follow up stun or silence, you may be better off leaving it at that instead of wasting your mana on extra spells. But you can also control the illusions to target a different opponent, or even push out the lane and farm.

Finally, the illusions can be very useful for heroes that tend to build a lot of auras, most notably a radiance, since your illusions will benefit from it, turning the enemy's radiance against them.

Soul catcher is extremely strong skill at all stages of the game, and arguably Shadow demon's only skill that scales well into the late game. However, it's search radius is fairly large and if you use it in the middle of a creep wave, odds are it won't hit your intended target.

Though it's tempting to always use it after disruption as part of your general combo, always use it by itself when you have any kind of opportunity as it tremendously increases your and your team's damage output and the duration lasts as long as the cooldown anyway.