Alternate method:
Create an ability based on Permanent Immolation (for a passive aura, or regular Immolation if you want to be able to turn it on/off). Make it as many levels as you can (I think 100 is the limit). Set the damage amount at each level to be equal to the current level, so level 1 = 1 damage per second, level 2 = 2 damage, etc. Works best if this is a unit ability rather than a hero ability.
Then, have a periodic trigger (every 1 second should be fine although you can tune it up or down) that sets the current level of your aura ability for the hero to the hero attribute you want to scale off of.
This is my go-to method for RPG hero abilities that scale off of a stat like this.