Bash

Bash is a passive base ability that grants a chance to stun the target upon attack.
Notes on Bash

Bash on Melee and Ranged units work differently. Also, illusions with Bash deals a special kind of stun.
Ranged bashes

Ranged bashes deal physical (Hero (Attack Type), Normal (Damage Type)) damage, while the stun is a magical effect and can be blocked by magic immunity. In addition, ranged bashes are blocked by Linken's Sphere. When multiple bashes trigger on the same attack, only the last one acquired will take effect. When both a critical and bash occur at the same time, if the critical was acquired before the bash, then the damage done on that attack will be (BaseDamage*CritMultiplier)+(BashDamage). However, if the bash was acquired before the critical, then the bonus bash damage will be skipped entirely and ther damage done on that attack will be (BaseDamage*CritMultiplier). However, the stun will be still applied.
Melee bashes

Melee bashes deal magical (Spells (Attack Type), Magic (Damage Type)) damage, while the stun is unblockable and unreduceable. When multiple melee bashes occur on the same attack, they will all deal their bash damage stacked but stun for the longest time(the effect is akin to having dummy units deal the bashes at the same time). Melee bashes stack with everything else, as it is considered a separate source from the unit's attack.
Bash on illusions

If an illusion bashes a target, the target will receive a "fake bash", where the stun animation will appear on the target but no damage is actually done and the unit is still capable of free action. However, if the target receives real bashes while being Fake Bashed, the real bashes will prolong the fake bash, not dealing any damage. Conversely, if the target was bashed by a non-illusion unit, then bashed by illusions, the illusions' fake bashes will turn into real bashes, prolonging the real bash as well as dealing damage. This is true for both ranged and melee bashes.
Stacking Bash

Bashes stack with diminishing returns by the formula

1-((1-A)*(1-B)*(1-C)...)

where A, B and C are the bash chances of the different items or skills.
Units that possess this ability

* Slardar's Bash
* Faceless Void's Time Lock
* Dwarven Sniper's Headshot

Items that grant this ability

* Cranium Basher
* Monkey King Bar

what's a good counter to bash? Is centaur good? His return.. how about nuke heroes

A good counter to bash would be either alot of evasion or an equal amount of disable.

You could also try to outbash them. Lol.

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just noticed tiny was really pretty good

Most permabashers are fragile during early and mid, thus strong nuker can harass them to make them fail late game.
Mass disablers cud also work against them.