Don't understand how Kha's unseen threat works

Hey first post on the forum, hope it's in the right spot.

I was hoping someone could explain how kha's unseen threat works. Im looking at the wiki, and I understand how his unseen attack gains a bonus 18 times through to level 18, but I don't understand at all the last bit - multiplying the per level bonus by "(10 + 5 / 10 / 15 / 10 at each level) (+ 50% AP) bonus magic damage and slow by 25% for 2 seconds".

Could someone that understands this walk me through it?

Ty