This is a Support or Carry build for Magnus,

The Quelling Blade is to help you jungle when you hit level 4 and have a bottle. this way you can farm quick enought to help support your team.

Magnus is a heavly needed support to initiate team fight and stun with his ult ( wich is almost the most important ult that is a game changer), therefor making wanting the blink dagger and the arcane boots a must as soon as possible,

After you get your core items Quelling Blade is no more necesary