Approximately how much damage does it add when activated to your auto-attacks?
I'm looking to add some information to my guide "Riftwalker Midwar Semi-carry" about the damage output.
Depends on the enemy's magic armor. If the enemy doesn't have any additional magic armor it increases it by quite a lot (like 25% or more I think), but the higher the enemy's armor gets, the lower the benefit of -armor. That's why Harkon's Blade is so good even on agility carries, cause regular armor can easily go up to 15-25 which pretty much makes a Shieldbreaker irrelevant, but with Harkon's the most you'll have to deal with is like 15 armor if the target has a headdress, unless of course it's a devourer.
Most agility carries will have at least 15 armor in the mid-late game, not to mention there'll be armor auras like Demonic and Nome's and stuff, but they usually wont even have a vestment because they'll be prioritizing damage items.
If you want the exact numbers you can just test it in a practice game. Look at the armor value of a hero and note how many percent of damage reduction they have against magic attacks, then apply the harkon's modifier and check the difference.
Harkons is never a good pick on an hero that is not particulary a carry or has carry items.
You are saying you was looking at a riftwalker semi carry build, I can tell you that it's not going to work out unless you have a Nullstone,Hellflower and harkons, since you will lack the attack speed, damage and regen.
For agility carries harkons is a situational pickup for when the enemy team has heroes like armadon or chtuluphant etc.
But then again they need the mana pool and best would be used in combination with some mana regen, as example hellflower on a madman even though it's better to have it on a ranged hero.
About the damage, I suppose you could say that harkons has one of the best damage output of the items in the game, if you want to see the exact statistics I suggest you go to the "balance discussion" section of the forums and check out the thread with harkons blade.
If you're against a tank heavy lineup pick up a hellflower for sure as a carry. That 20% extra damage + silence basically makes armadon unable to spam his spines and lego unable to spin. Then you get that harkons and you're able to pound away at them totally negating all that physical armor they build up expecting you to go some physical damage carry item.