Passive: Varus' attacks deal an additional 7 / 12 / 17 / 22 / 27 (+30% of ability power) magic damage and apply a Blight stack for 6 seconds (max 3 stacks). Varus' other abilities detonate Blight stacks dealing 3 / 3.5 / 4 / 4.5 / 5% (+2.5% per 100 ability power) of target's max health as magic damage per stack. Detonating Blight on champions and epic monsters also reduces his basic abilities' cooldowns by 12% of their maximum per stack.
Active: Varus' next Piercing Arrow deals an additional 6−14% missing health as magic damage (depending on level), increased up to 80% missing health damage depending on charge time.
Runaan's proc this?
i suppose so, its an on-hit effect