This is a datadriven + lua ability that will apply the invis state and texture, with some extra particles and effects.

(Some effects might not be from this example, as this video shows the Assassin hero, find it in the repository later on)

KV:

 "assassin_walk_the_shadows"
 {
  // General
  //--------------------------------------------------------------------------------------------------
  "BaseClass" "ability_datadriven"
  "AbilityTextureName"	"assassin_skill1"
  "MaxLevel" "7"
  "AbilityBehavior"	"DOTA_ABILITY_BEHAVIOR_NO_TARGET | DOTA_ABILITY_BEHAVIOR_IMMEDIATE"
  
  // Stats
  //--------------------------------------------------------------------------------------------------
  "AbilityCastPoint"	"0"
  "AbilityCooldown"	"5"
  "AbilityManaCost"	"6 8 11 14 18 22 26 30"
     
     // Damage.
     //-----------------------------------------------------------------------------------------------
     "AbilityDamage" "25 150 300 600 1100 2000 3300 5000"
     "AbilityUnitDamageType"	"DAMAGE_TYPE_PHYSICAL"

     // Special
     "AbilitySpecial"
      {
      "01"
       {
    "var_type"	"FIELD_INTEGER"
    "duration"	"25"
       }
      "02"
       {
       "var_type"	"FIELD_INTEGER"
       "bonus_movespeed"	"35"
       }
      "03"
       {
       "var_type"	"FIELD_INTEGER"
       "bonus_damage"	"150 300 525 840 1260 1900 3000 4500"
       }
      "04"
       {
       "var_type"	"FIELD_FLOAT"
       "stun_duration"	"3.25"
       }
      }

     "precache"
     {
      "particle"	"particles/units/heroes/hero_templar_assassin/templar_assassin_meld.vpcf"
      "soundfile"	"soundevents/game_sounds_heroes/game_sounds_templar_assassin.vsndevts"
     }

     //----------------------------------------------------------------------------------------------
  "OnSpellStart"
  {
   "FireSound"
   {
    "EffectName"	"Hero_TemplarAssassin.Meld"
    "Target" "CASTER"
   }
   "RunScript"
   {
    "ScriptFile"	"abilities/assassin.lua"
    "Function"	"walk_the_shadows_cast"
   }
  }
  "Modifiers"
   {
   "assassin_walk_the_shadows_buff"
    {
    "IsBuff"	"1"
    "Duration" "%duration"
    "EffectName"	"particles/units/heroes/hero_templar_assassin/templar_assassin_meld.vpcf"
    "EffectAttachType"	"follow_origin"
    "OnCreated"
    {
     "FireEffect"
     {
      "EffectName"	"particles/units/heroes/hero_bounty_hunter/bounty_hunter_windwalk_smoke.vpcf"
      "EffectAttachType" "follow_origin"
      "Target" "CASTER"
     }
    }
    "Properties"
     {
     "MODIFIER_PROPERTY_MOVESPEED_BONUS_PERCENTAGE" "%bonus_movespeed"
     }
    "States"
     {
     "MODIFIER_STATE_INVISIBLE"	"MODIFIER_STATE_VALUE_ENABLED"
     "MODIFIER_STATE_NO_UNIT_COLLISION"	"MODIFIER_STATE_VALUE_ENABLED"
     }
    "OnAttackLanded"
     {
     "RunScript"
      {
      "ScriptFile"	"abilities/assassin.lua"
      "Function"	"walk_the_shadows_attack"
      }
     "Stun"
      {
      "Target"	"TARGET"
      "Duration"	"%stun_duration"
      }
     }
    "OnAbilityExecuted"
     {
     "RunScript"
      {
      "ScriptFile"	"abilities/assassin.lua"
      "Function"	"walk_the_shadows_interrupt"
      }
     }
    }
   }
  
 }

Lua Scripts:

function walk_the_shadows_cast( event )
  event.ability:ApplyDataDrivenModifier(event.caster, event.caster, "assassin_walk_the_shadows_buff", nil)
  event.caster:AddNewModifier(event.caster, event.ability, "modifier_invisible", {duration = 25}) 
  
end

function walk_the_shadows_interrupt( event )
 event.caster:RemoveModifierByName("assassin_walk_the_shadows_buff")
 event.caster:RemoveModifierByName("modifier_invisible")
end

function walk_the_shadows_attack( event )
 
 event.caster:RemoveModifierByName("assassin_walk_the_shadows_buff")
 
 ApplyDamage({ victim = event.target, attacker = event.caster, damage = event.ability:GetAbilityDamage(), damage_type = event.ability:GetAbilityDamageType(), ability = event.ability	})

end

The line that takes care of applying the “transparency” is AddNewModifier with modifier_invisible.

Credits to my buddy igo that made this ability for The Black Road project which is still WIP but there are some interesting stuff we made already.