So most people, when they play magmus, go for boots and a power supply or something and then rush a portal key, then spellshards/ult stick whatever. This makes him a solid initiator with a lot of focus on his ult, which is good, but I've been doing things differently, and it's been working really well in 1700-1750 level games. I dunno how it would work higher up because I haven't been there in a while.
Anyhow, I build like this:
Starting items -- runes, 2 mana potions, health potion, +2 armor ring, 2 minor totems
Then I finish ring of the teacher, get an iron shield, finish power supply, and steamboots.
Then I work on shaman's headdress and, depending on how my game's going, I finish a HotBL or go straight for heart.
The mana from ring of the teacher and power supply gives me enough to roam and cast my stun without going back to base.
The rest makes it really difficult to kill me, so I can tower dive when I gank, or gank 3 people at once.
Usually when people see a magmus stun their teammate they unload all their abilities on him, and this tankiness lets me absorb that damage and be able to stun again once it comes off cooldown.
This build turns Magmus from a fragile initiator into a really beefy tank. Because S2 buffed his stats last patch, he can play this role now.
My skill build when I do this is generally Stun, Steam, Stun, stats, Stun, ult, Stun, stats.........ult...stats... other stuff
Here's an example replay, I ended up getting 422 gpm and 717 xpm, sure it's a pubstomp but I don't usually stomp this hard with Magmus. 32293314