Artio Support (Defensive/Sustain)

Alright, so here's the more in-depth feedback regarding items:

You don't list a start, but I would go: Guardian's Blessing, Shoes, 3x Healing Potions and Mana Potions. Sure you have healing in your kit, but it takes a bit to come online.

Full build with boots:
Shoes of Focus: power, mana, and CDR, all of which Artio can make good use of.
Stone of Binding: its passive is great if your ADC follows up with your initiation. Since it's considered a bridge item, you would sell this for something else before you bought the Elixir of Speed and traded out your shoes.
Sovereignty: physical protections and HP5 to help with your healing, plus it's an aura item that provides prots and HP5 to nearby allies.
Void Stone: Artio's an aggressive guardian, so the extra magical protection, power, health, and protection reduction really help her; it'll also help any magical gods on your team.
Gauntlet of Thebes: good health, double prots when fully stacked, and HP5. Note, you can get this instead of sov 2nd if you like depending upon how the match is going.
Elixir of Speed: grab this after your build is finish, sell your shoes, then buy your 7th (or 8th) item.

After you get the above items, it's time for situational ones (you can skip sov if you don't need it):
Pythagorem's Piece: power, health, cdr, lifesteal, and its aura gives your team lifesteal. This is a great item for those team fights provided you don't need something else.
Oni Hunter's Garb: This is mainly for the passive: 9% damage mitigation is nothing to sneeze at though the other stats are great as well.
Spirit Robe: even though the mitigation is situational, it's still a great item with good stats all-around, plus its passive is on a shorter cooldown than Mantle of Discord. And it stacks with Oni.
Heartward Amulet: a very situational item you should only get if you're facing 4+ magical gods; otherwise there are better magical protection items like Oni.
Emperor's Armor: if you're having trouble taking down objectives or defending your own, this can help. I say it's pretty niche, but it depends on the matchup and how the game's going.
Hide of the Nemean Lion: for dealing with gods building crit. Be careful because they may stop targeting you once they figure out you built this.
Midgardian Mail: great against AA gods who are not building crit. Don't buy this and Hide together because you want crit gods to hit you as much as possible so you can reflect as much damage as you can.
Winged Blade: good if you're facing a lot of gods with slows in their kit. Note that its passive does have a fairly long cooldown.
Witchblade: works pretty well with Midgardian, but I consider it a niche item on Artio since she doesn't have very good attack progression.

Relic choices depend upon matchup, but these are always good choices:
Magic Shell: helps defend you and your teammates against AA gods and those with executes.
Heavenly Wings: helps retreat or chase a low enemy.
Belt of Frenzy: a decent 2nd relic for helping take down objectives.

Situational relics:
Horrific Emblem: mainly used to counter an enemy who bought wings.
Blink Rune: so you can blink in and stun grouped up enemies. Make sure your team knows what you're doing before you blink in.
Phantom Veil: for countering Odin's, Ymir's, and Cabrakan's walls. I'd only get this 2nd if you really need it because it's very niche.

Remember that CDR is capped at 40% and Potion of Magical Might gives you 10%. You can over cap CDR, but you're not building efficiently then.

Thoughts? Also, I haven't voted because I'm waiting to see how you update this guide. :)

I would probably get rid of mystical mail. In 99% of cases you do not need this as a support.

I have some serious concerns with your build, Usatells. I'll provide my feedback if you're interested in hearing it. Ultimately this is your build and you can choose what items you want but I cannot recommend it in its current state.

pythags, typhon and mystical mail?