Knowing that Magnus has one of the best crowd-control skills in the entire game,I would suggest a build similar to Tidehunter's.
I personally play him like this.
Early on get a Soul Ring (Shockwave harass and good health regeneration)
Next off I get Arcane Boots combined with a Dagger.
At this point you can sell off your Soul Ring and start building a Refresher Orb.
After the Refresher I usually buy a Heart of Tarassque.
But I rarely get that far in if I've got a good team.
For the build,I max out Shockwave first,Skewer second,Empower Last.
Get the Ultimate at the standard levels,leave stats for last.
The reason I don't get any damage items is because Empower will provide me
with an awesome damage boost as well as a cleaved attack.
Whatever you do,don't go for the stupid Battlefury build.
Breaks my heart to see people doing that.