Why Magnus?

Magnus seems to be a very underrated hero in this game. However, given the correct conditions, he can be very devastating. He's a caster DPS who's damage alone is somewhat mediocre, except he can nuke from a safe distance, is very mobile for a strength hero, and brings empower to the table, which buffs your team's rightclick by a very large margin.

When should Magnus be picked?

Only pick Magnus when he would be the sole caster DPS of your team (that is, everyone else is a tank or a physical damage right-clicker) because it would allow the rest of your team to get the maximum benefit of your aghs upgrade (empower aura). If someone else already picked a caster DPS, don't pick Magnus, go pick some other hero.

Skill Build

Shockwave > Empower > Skewer > Stats

Shockwave is your main damage ability. At max rank it will deal 28000 damage, so it's outclassed by other dedicated nukers like Lina or Leshrac, but it's easily spammable from a safe distance, especially when you get the items I will talk about later on.

Empower is the skill you want to max out next. Combined with aghs you will boost your team daamge by over 75% (not counting the cleave of course).

Skewer is maxed third. You're not usually going to be using this ability for damage, since you want to avoid danger and keep yourself alive so that your allies get as much benefit from the empower aura as possible. However, you want to max this out because it substantially lowers the cooldown of skewer, keeping you constantly mobile. Be careful of dragging bosses along with you if you need to use this for escape however.

Reverse Polarity sucks. Don't level this ability, as you're likely going to get yourself killed before you get it off.

Items

Midas Hammer > Force Boots > Aether Lens > Octarine Core > Veil of Discord > Aghanims' Scepter > Upgrade your other items, in any order.

Get Aghs ASAP! You will boost your team's damage by a very large margin. Make sure to stay near your physical DPS at all times so they can reap the maximum benefits (hence the reason why you get blink boots and skewer).

The other items are generic caster DPS items.

Playstyle

Stay near your team so that they get the empower aura, but far enough so you don't get cleaved down by bosses. This cannot be stressed enough. This is especially prudent in summoner waves as your dps will be able to cleave summons.

Make sure enemies have the veil debuff before you start spamming shockwave. When in a sticky situation, don't be afraid to use skewer as an escape, especially if force/blink boots is on cooldown.