If you just want to know how much damage you do while Omnislashning and don't care about how it works technically, read the basics.

1. Spell facts
2. Trigger basics
3. History and other stuff 

1. Spell facts

Yurnero moves around the battlefield, slashing many enemies. Yurnero becomes invulnerable while Omnislashing.

Level 1 - Attacks 3 times.
Level 2 - Attacks 5 times.
Level 3 - Attacks 8 times.

Cooldown: 130/120/110 seconds.

Level 1: 200 mana.
Level 2: 275 mana.
Level 3: 350 mana.

Casting range: 450 units.
Jump range: 575 units.
Total casting time: 0.3 seconds (including Juggernaut's casting point).
Extra: Ministun on the first target.
Duration: 1.2/2/3.2 seconds on level 1/2/3.
Slash duration: 0.4 seconds
Slash damage: 150 - 250 (Attack type Hero, Damage type Normal. Reduced by armor, not blocked by magic immunity)
Can attack while slashing: Yes

Can cast on: Non-ancients.
Can jump to: Non-ancients.

No buffs (including Berserk from Mask of Madness) will be removed when activated since it only makes him invulnurable, not magic immune. 

2. Trigger basics

First of, the tooltip is a bit misleading since the spell itself don't count the number of attacks made by Juggernaut. The spell is completly triggered and works by moving Juggernaut around to different enemies dealing some random triggered damage. It is like 3/5/8 Blink Strikes that deal random amount of damage and target random enemies.

1. Ability triggers starts.
2. Blade Fury is disabled.
3. Juggernaut is made invulnurable and unselectable (if you select him, a trigger will directly unselect him).
4. Juggernaut is moved to the selected target, and is turned so he faces it.
5. The trigger issues an attack order to the target.
6. The trigger deals 150-250 standard hero damage to the target.
7. The attack animation is played.
8. Wait for 0.4 seconds.
9. If all slashes have been made, go to 10. Else, choose a new random target and go to 4.
10. Blade Fury is enabled.
11. Juggernaut is made vulnurable and is selected.

The amounts of "attacks" said in the tooltip refers to the amount of triggered moves/slashes, not on how many actual attacks he does. So, while Juggernaut is moving around (and the trigger deals damage) he will also try to attack the targets with his standard attacks. The problem is that the attack is interrupted on each move he does (each slash in Omnislash) so he can't attack freely. On these extra attacks he can get all added effects attacks can do (lifesteal, bash, critical strike, maim, corruption, etc), but not on the triggered damage.

The amount of extra attacks that will be made during Omnislash is based only on how fast you are attacking, namely the amount of Increased Attack Speed (IAS). Here you have a graph which shows how many extra attacks you will get for a certain amount of IAS. 


So for example, if you got 140% IAS and do a lvl 2 Omnislash you will get 3 extra attacks which deal your standard attack damage.

Total damage would be: 150-250 * 3/5/8 + AttackDmg * NumExtraAttacks 

3. History and other stuff

Before 6.46, Omnislash was coded differently and was a lot more random in how many extra attacks you got. In 6.46 it was fixed and in 6.46b tweaked again. Now the only thing that defines the amount of attacks, is how fast you attack (that is, IAS).

I also made a big mistake, all my graphs have showed a too small value due to some errors in my script. It should be fixed now, but to be sure I'll post my script here and if anyone notices an error you can PM me or post a reply here. The language is PHP, but it should be readable by anyone with minor programming knowledge.

The Code

$wait = 0.4; // set the time for each slash

for ($ias=0; $ias<=400; $ias+=20) {
$dmgp = 0.33/(1+$ias/100); // set the damage point
$cooldown = 1.7/(1+$ias/100); // set the attack cooldown
$c = 0; // set the start cooldown to 0

for ($h=0; $h<8; $h++) { // run this for each slash
$left = $wait; // set the time left of the slash

while ($left > 0) { // try to attack if there is some time left
if ($c + $dmgp < $left) { // if there is enough time left for an attack
$left -= $c + $dmgp; // decrease the time left
$c = $cooldown - $dmgp; // set the current cooldown to attack cooldown - damage point
$attacks[$ias]++;
} else {
// if an attack cant be made (not enough time), just decrease the cooldown
$c = max(0, $c-$left);
break;
}
}
}
}

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