ALWAYS buy blink, the second relic is open for debate. I prefer the shield because it helps my team survive here and there.

Notably, all items give an aura. We like this for two reasons, it gives Ares free Magic Power from the passive, and it will either help your team or hurt the enemy team. Since the map is small, you'll usually be able to effect every player in the teamfight with the auras. Remember to add +30 Magical Power to all items because of his passive.

No boots - Typically you'd buy Boots of Focus (CDR), but I find some of the stats unimportant. The mana is useless most of the time. Sometimes you do run out of mana in a sustained team fight, but generally you won't notice the lack of mana if you're somewhat conservative. Witchblade gives Health (best stat) and the aura not only makes physical enemies worse, it also gives you 30 Magic Power from Ares' passive. In the end you trade 8% move speed, 25 Magic Power, and mana for HP, and a debuff for the enemy team.

Gauntlet of Thebes - Balanced stats, effective against all enemy teams. Getting it early allows for easy stacks.

Sovereignty & Heartward Amulet - nothing to discuss. Swap the Sov. and HA order if the enemy team is Mage heavy.

Pythagorem's Piece - it has stats we like and it gives stats your teammates like. I get this before Void Stone because it helps everyone on your team (instead of mages only), plus the protections from Void Stone isn't critical at this point in the game.

Void Stone - 90% of the time this will be your last item. It has stats we like plus it boosts your mages' damage. I'll explain the exceptions to this item below, but when in doubt get this item.

Situational Builds:

If the enemy team is physical or magical heavy, do not follow the build verbatim. Remove either Sovereignty or Heartward Amulet and replace it with something below. Always counter the enemy team with the appropriate protection items. Damage items are last, if ever.