This guide for swain can be taken as an aggressive build. It focuses mainly on swain's ability with short bursts of massive damage and his ability for stopping enemies in their tracks.

Pros
a tanky mage who can come back into combat easily by farming with his ultimate.
slow, stun.
joke is amazing.
damage output is over time making it hard for people to successfully escape.
good mid to late game.
can gain lots of health on the run.

Cons
early game kinda sucks.
not an escape artist.
long cooldown on stun.
heavy mana user.

the extra mana from the crystal is very important early game it allows for longer laning with in turn allows for more farming, it also acts as a fail safe in case you get a little to aggressive with your spells. unfortunitely because of swains early game flaws until he hits level 6, the potions become extremely helpful. getting the catalyst early offers the most benefit not just from the extra health and mana but from the level up activation as well. Which allows for more laning time as well as more mana for harrassing your opponent. grabbing the sorcerer's shoes and rod of ages is a good way to make swain more versatile and more threating. after that the order doesnt matter but getting the tear of the goddess first is a good idea. Then the deathcap, archangels, will of the ancients, and rylai's should be in there in any order you like i tend to go archangels first because of the mana boost and extra power then into the rylai's for the extra survivability and slow. every caster needs a deathcap, as for the will if you find you have not alot of stuns or slows on the enemy team (or your team if mostly ability power based) its a good item, otherwise i might switch it for the banshee's veil just for the extra tankyness.

taking utility masteries allowes for a better mana pool, cooldowns, and movement aswell as lets you flash and exhaust more often. then the offensive masteries allow for more damage for bursts as well as more usage of your spells.

My choice in runes is actualy fairly standard for a mage. cooldown reduction glyphs, magic penetration marks, ability power seals, and magic resistance quintessences. which will make his abilities more castable awith more damage and will make him more resilient to damage(not physical damage though).