Silencer – Unlimited Power: Glaives of Wisdom also permanently steals 2 intelligence from an enemy hero that dies within 925 range.
Passive is Non-retroactive.Riki – Two for One: Aghamin’s Scepter applies a double strike during Tricks of the Trade.
Tiny – How did I get here? (RIP): Buying back during toss spawns the hero at the end of the toss.
Pangolier – Bitchslap: Swashbuckle applies a fifth 5th hit if Pangolier travels through an enemy during the dash.
Axe – Dazzle Deez Nutz: An ally blessed with Shallow Grave, no matter how close to death, cannot die while under its protection, except by Axe.
Tusk – Snowballin: Tusk can cast abilities while inside Snowball.
Huskar – Attacker or Caster? Manually casting Burning Spears gives the ability an extra 50 attack range.
Bane – Sweet Dreams: The first frames of Nightmare make the unit untargetable and damage immune.
Morphling – Gamebreaker: Discover an interaction that in some way breaks the game.
Nature’s Prophet – One with Nature: Has the ability to farm jungle creep at a slow rate from a cliff.
What are some others that I missed?
Source: https://www.reddit.com/r/DotA2/comments/nmdsxa/hidden_dota_2_passive_abilities/
