This guide is mainly catered towards Arena, but also applies to Clash and Siege and to some extent Joust. The main focus is to build as much penetration and damage amplification items as possible, to ensure your 2 > AA > 1 or 2 > AA > 4 combo instantly kills any squishy targets. I've found this build path to be the most effective when it comes to maximizing damage output. Downsides of this build may be lack of mana sustain and CDR. But keep in mind this build is only a general guideline so you can swap out items to fit your own playstyle.
Next to the build, I will also be mentioning gods Discordia's levelling order and matchups at the very end of this guide.
This is my main Arena build for Discordia. There is a focus on penetration and damage amplification to maximize damage output.
Shoes of the Magi - Finish these boots after clearing the buff camps on the left side and clearing about two minion waves.
Spear of Desolation - Your next item should be Spear of Desolation. Quite expensive, but offers all the stats Discordia needs: CDR, Flat penetration and a good chunk of power. If you get this item online by the time the buffs come up again, you're doing a great job in terms of farming.
Divine Ruin - At this point you have 50 flat penetration which is enough to deal true damage to defenseless squishies. You're going to notice a huge boost in damage at this stage of the game. Very useful item as it is cheap and also provides an anti-heal effect. This item can be swapped out for another item if they have no healing at all (see alternative item options below). BONUS TIP: Divine Ruin procs on the little starbits of Discordia's 1.
Obsidian Shard - Your tank shredder item. Get this and even tanks will fear your damage. Can be swapped out for another item if they have zero magical protection items (see alternative item options below).
Shaman's Ring - Excellent item for Discordia as Shaman's passive synergizes very well with her. The main damage combo gets amplified because the 2 sets up your 1, which means your 1 will hit like a train. Shaman's Ring also procs on the little starbits from your 1 which makes your next combo deal an immense amount of damage. Lastly, the initial hit of your ultimate applies Shaman's passive, which means the detonation will also hit 10% harder. The movement speed is also a nice bonus as Discordia is very squishy and the movement speed helps her get out of sticky situations.
Rod of Tahuti - Same deal as Shaman's Ring, difference being it has a +25% damage increase instead of +10% and it only procs on targets below 50% health. When you get this item online you legitimately two-shot squishies with your basic combo.
+20 Physical Power +25 Magical Power ROLE QUEST: DAMAGE Damage lane minions and Enemy gods to gain stacks. Dealing 100 damage to Lane Minions provide 1 Stack and Dealing 100 damage to Enemy gods provide 3 Stacks. At 100 Stacks Attacker's Blessing Evolves, gaining 10 Penetration.
Shoes of the Magi
Spear of Desolation
PASSIVE - If you receive a kill or assist on an Enemy God all of your cooldowns are reduced by 2 seconds.
PASSIVE - Enemies hit by your abilities have 40% reduced healing and regeneration for 8 seconds.
Rod of Tahuti
PASSIVE - Basic Attacks and Abilities gain 25% additional Magical Power against targets below 50% Health.
Alternative itemization options that might fit your playstyle better. Some of these items can also be used as situational counter-pick items.
CDR and Mana Sustain
Chronos Pendant - Very useful item if you want to spam your abilities more often. If you don't need one of the items from the main build, consider getting more CDR with this item.
Book of Thoth - Good item if you run into mana problems. Personally I don't feel like this is an issue in Arena, but it is by no means a bad item for Discordia.
Doom Orb - Usually I don't recommend picking this up as this item requires you to be very fed because it's so expensive, but it is always a great option if you can afford it.
Bancroft's Talon - This item is an amazing item and works really nicely on most mages, including Discordia. Pick this up if you feel like you need more health sustain and cheap power.
Polynomicon - Polynomicion is extremely useful when the enemy team is mainly composed of squishy gods. It's an extremely fun item to use. It synergizes well because your 2 sets up for an unmissable Poly AA followed up by your 1 or ultimate. Get this in combination with Rod of Tahuti so your 2 > Poly AA will put them below 50% health, and smack them with a +25% damage amplified Unruly Magic and watch them drop.
Soul Gem - Great item if you need a little bit more survivability. The CDR is very helpful as well.
Burst and Anti-Health
Soul Reaver - Super useful item if the enemy has a ton of health but little defenses. Expensive but worth it. BONUS TIP: Soul Reaver procs on the little starbits of Discordia's 1.
Ethereal Staff - Niche item, it is very effective against high health targets since you get a percentage of their health. Since it is essentially the defensive version of Soul Reaver, I recommend picking this up if you need a little bit more survivability.
PASSIVE - Every 10s the Pendant activates, subtracting 1s from all of your abilities currently on Cooldown. The initial countdown will not start until you leave the fountain.
Book of Thoth
PASSIVE - You permanently gain 10 Mana per Stack, and receive 5 Stacks for a god kill and 1 Stack for a minion kill (max. 75 Stacks). 6% of your Mana from items is converted to Magical Power. At 75 stacks this item Evolves, gaining 3% extra Mana to Power conversion.
PASSIVE - You gain additional Magical Power and Lifesteal scaled from missing Health. This caps at 100 power and 20% Lifesteal at 25% Health.
PASSIVE - Using an ability gives your next Basic Attack within the next 8 seconds +75% of your Magical Power as additional Magical Damage. The effect can only apply once every 3 seconds.
PASSIVE - On successful hit of an Ability you gain 1 stack. At 4 Stacks your next Ability that damages an enemy God will deal bonus damage equal to 30% of your Magical power to each God hit, and will heal yourself and allies within 20 units for 40% of your Magical Power and will consume the 4 stacks.
PASSIVE - Your abilities deal an additional 2% of the target's maximum Health as Magical Damage. If the target has over 2000 Health, your ability bonus damage scales up. This effect reaches a maximum of 7% Maximum Health damage at 2750 Health. Subsequent hits on the same target do half the bonus damage for the next 3s.
PASSIVE - Whenever you damage an enemy god with an ability you steal 8% maximum mana as well as 6% maximum health from the target. The Stats remain stolen for 45s and targets affected can have multiple Buffs and Debuffs at once. This can only occur once every 15 seconds.
Focus on levelling the 1 for minion clear and burst damage. Then the 2 for even more damage and lastly the 3 to get your abilities up more often. Put a point into your ultimate whenever available.
Discordia is very good against immobile gods since they have no escape from her Strife. They are forced to eat the entire combo unless they use their Aegis or Purification Beads.
Discordia is weak against gods that can rush her down and harass her. Her main enemies are assassins and warriors.
Discordia synergizes very well with gods that can help her setup her main damage combo. Gods with CC, control and protective abilities pair very well with Discordia.
Discordia doesn't do very well with gods that have displacement abilities since they can mess up her combos. A mistimed wall can also ruin Discordia's ultimate.