What is ally hero deny in DOTA 2 and why do you need it?
What is deny?
Deny means the killing of an ally's creep or hero. Or destroying an ally's tower. Denying creeps and towers is a common thing, but many players are not aware that heroes can also be denied.
Why do you need to deny heroes?
To prevent enemies from getting gold and experience.
How to do it?
You can deny ally's heroes only under certain enemy spells.
To deny an ally hero in DOTA 2, just press "A" and click on an ally. Deny is possible when the ally hero has less then 25% health.
When you can deny an ally's hero?
When he is under:
DOOM (ultimate 
Doom)-
Venomous Gale (first skill 
Venomancer) 
Shadow Strike (first skill 
Queen of Pain)
Heroes, that can deny themselves
Techies - 
Suicide Squad, Attack!
Pudge - 
Rot
Alchemist - 
Unstable Concoction
Abaddon - 
Mist Coil
Pugna - 
Life Drain when used on an ally
Example:
An ally hero deny happens in pro-scene as well. 
Team Secret and 
LGD played against each other during group stage of Dota 2 Asian Championship. 
Arteezy got Doom and was going to die. So, Arteezy killed himself with the help of his own wolves.
Also you can deny ally heroes with spells:
Bane - 
Nightmare- Kill with ally creeps that are being controlled by Chen, Enchantress or any hero with Helm of Dominator.
- Creeps created by Nature's Prophet, Lycan, Beastmaster and etc.
It's interesting:
- You can deny yourself with
Bloodstone. Just activate it! - You can deny
Phoenix during 
Supernova if he has less then 50% health. - Any hero can be denied at any time by neutral creeps or Roshan.
Source: gamepedia.com
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