
Disruption
Banishes the targeted unit from the battlefield for a short duration. Upon returning, two illusions of the banished unit with bonus base damage are created under Shadow Demon's control.
Abilities, roles and builds with an explanation for every purchase.

HERO GAME PLANI save an ally with Disruption and restrict a dangerous target with demonic disables.
Spewing clouds of demonic poison to damage enemies or scout the battlefield, few dare to approach Shadow Demon. Banishing an enemy as his allies gather for the kill, he can inflict demonic curses as he summons traitorous illusions to turn enemy strength against itself.
My recommendation is a starting point. We choose another position together with the team and for its needs.
Help the lane, provide vision and keep the ally alive first; expensive upgrades follow the necessary team purchases.
I contribute through laning, control and movement. I adapt the expensive core route to my budget.
This is an editorial starting plan based on hero mechanics. It is not presented as a match I have already tested or a popularity ranking.
Here is my initial plan: I do my role's job first, then choose the items the team needs. I adapt the order and expensive purchases to the opponent.

Provide mana for laning and regular ability use.

Lets you use targeted abilities and items from a safer distance.

Adds a protective tool and hides the chosen ally; invisibility does not replace escaping detection.

Moves me or an ally out of danger; direction depends on the target's facing.

Unlocks or upgrades Demonic Purge. The exact effects are shown below in the abilities section.

I do not assign one ‘correct’ minute to every purchase. I look at my gold, the next fight and the problem: am I missing mana, target access, control or protection? If an expensive slot will arrive too late, I get a useful answer earlier.
Original Valve icons and descriptions. Cooldowns and mana are shown by level from the data snapshot; talents and upgrades can change the values.

Banishes the targeted unit from the battlefield for a short duration. Upon returning, two illusions of the banished unit with bonus base damage are created under Shadow Demon's control.

When the ability’s target takes damage, all enemies within a radius of 675 of the target, including the target itself if it is an enemy, also take a portion of that damage. Can be cast on enemies or allies.
Whenever the target takes damage, all enemies, including the target itself if it is an enemy, within 675 range of the target also receive a portion of that damage. Can target enemy and allied units. Effect is paused if the target is hidden by Disruption.

Launches a cloud in the selected direction, damaging and poisoning enemies. The first 5 hits on the same target multiply the poison’s base damage by 1/2/4/8/16, while each subsequent hit adds 60 damage.
Deals damage in a line, and afflicts enemy units with a poison effect. The poison deals 1/2/4/8/16 times the stack damage based on the number of stacks on the target, up to 5 stacks. Additional stacks cause 60 damage each. This deferred damage is dealt when Shadow Poison's duration is expired, or the Release sub-ability is used.

Purges the target, removing all positive effects and slowing it. The target’s movement speed gradually returns. When the effect ends, speed is fully restored and the target takes damage.
Purges the target enemy unit, removing positive buffs, and slowing the target for the duration. The unit slowly regains its speed until the end of the duration, upon which damage is dealt. Units under the effect of Disruption can still be affected by Demonic Purge. DISPEL TYPE: Basic Dispel
Replaces the cooldown with 2 charges. The ability applies break to its victims.
These abilities are not necessarily available at the same time. I check the condition beside the name: innate ability, another form or an Aghanim purchase.

Purges an allied target, continuously removing all negative effects for a duration. Heals the target when the effect ends. Works on units affected by Disruption.
Cleanses the target allied unit, removing negative buffs for the duration. At the end of the duration, the unit is healed. Units under the effect of Disruption can still be affected by Demonic Cleanse. DISPEL TYPE: Basic Dispel

Shadow Demon's Attacks apply a stacking debuff to the enemy for 8 seconds that increases their damage taken.
Shadow Demon's Attacks apply a stacking debuff to the enemy for 8 seconds that increases their damage taken.
These are fields from the official source. The hero's final values in the client account for attributes and other active mechanics.
Items, slang and real episodes help connect abilities with what happens in a match.