Do not take Phase Rush on Swain

Instead, take Electrocute, and switch out one of your summoner spells for Ghost.

I have been playing plenty of Swain lately, and I've come to realize that he is someone that needs to establish lane control fairly early on (I mean by like lvl 2). Phase Rush doesn't help this.......at all. When you're trying to harass in lane, landing a E, passive, Q can do a hefty chunk of damage, and will assuredly force your opponent to down a pot after just 2 successful attempts. Phase Rush is meant to help during his ult so he could keep up with his targets....but it doesn't even do it's job here. It doesn't last long enough, and it's CD is long enough that by the time you can proc it again, your ult is basically over. However, in lane, it's CD is short enough, while your CDS are long enough that you use it to just get away from your opponent. Someone like TF has low enough early CDs that he can use Phase Rush properly.......but Swain doesn't have that.

Electrocute, on the other hand, helps so much more. You can go from forcing your opponent to down a pot in 2 successful combos, to just 1. Then, to help keep up with your targets while in ult, you can take Ghost. Not only does it last so much longer and gives so much more speed than Phase Rush, it's CD is low enough that come the time you would need to use your ult again, it will be waiting for you.

Now, you might be asking "What about teleport, or ignite?" Well, teleport is good if you know you're going to have a rough lane....but even then, I would still recommend Ghost over it, because it can help you far more in the mid game teamfights. As for ignite.....you're not a burst mage. You're not going to bring your opponents within an inch of their life many times throughout the game....you're either going to kill them....or your not. Simple as that.