Hero ablity based on a number of attacks

[Gojira2000]

How would I make a hero ability (in triggers) for lets say this

"hero gets 20% lifesteal for the next six attacks when ability is activated"?

 

[pOke]

You activate the ability.

Add a hidden spell book with vamp aura/Mask of Death

A unit is attacked

increment a variable from 0 to 6, when the value == 6 remove the hidden spell book.

Alternatively you could use a DDS and just set the live of the unit with some basic math.