Many ask: How should I build Agni?
Answer: Survivability and CC (Crowd control)
How I personally build Agni really depends on the enemies god selections. Agni is both viable in a side lane or mid lane. I tend to play him middle lane as compared to side, so roughly 9 times out of 10 I will go against a magic power god middle.
Items I build in order (Against a magic user mid): 1st:Magi boots/Reinforced Boots (Depending on CC capabilty on the enemy team)
2nd: Void stone/Warlock Sash (Depending how many magic users the enemy team has)
3rd: Gem of Isolation (A MUST)
4th: Obsidian Shard/Rod of Tahuti (Depending on how much magic resist the enemy team has)
5th and 6th items tend to be a toss up due to their team comp. and items the enemy has.
If I go Obsidian Shard 4th, I would go Rod of Tahuti 5th, and vice versa. As a final item, you may choose whether to go more sustain (Hide of Leviathan) or more damage (Pythagorem's Piece)
The core item of building a quality Agni would have to be 'Gem of Isolation'. This is a great when it comes to surviving or keeping a team slowed constantly with the juggling of your abilities.
How do I skill Agni?
Passive: Combustion - After hitting with 4 basic attacks, Agni will gain a buff. On next cast of Flame Wave or Rain Fire, all enemies hit by those abilities will be additionally set ablaze, taking 5 (+10% of your magical power) magical damage every .5 s for 3 s. Although this passive may not seem strong, it's actually quite useful middle and end game.
When leveling up Agni's skills, it's pretty obvious what you want to max first. Maxing 'Path of Flames' over 'Flame Wave' typically seems better in my opinion. The reason being: Even though the cooldown is overall higher than Flame Wave, you will not always be able to be in such a close range to get that damage off. Path of flame is great for pursuing targets as well as evading them. Although many are not forced to walk into your 'Path of Flame' skill, you can still aim it well and run over the enemy target.
Your 'Path of Flame' skill is great with it comes to combining with your 'Noxious Fumes'. You can either go for the safe bet of dropping your 'Noxious Fumes' on the enemy target(s) to make sure you land your stun, or take the alternative route (Which i do 90% of the time), and dash through with 'Path of Flames' and being able to target where you want to land your stun (Noxious Fumes).
One of the greatest things about Agni is his ultimate ability: Rain Fire. The obvious reasons: No mana cost + Long Range. This ability is just great when it comes to the god. This is great for lane sustainability as well as picking others off or poking at them when trying to keep them at bay. During team fights it's great to poke at the enemy, keep them slowed (long range), or just pump out some burst damage in the process. (Can proc. 'Combustion' effect)
Flame wave: One of the more basic abilities in smite, this can be one of Agni's farming and harrassing abilities. After gaining your passive through auto-attacking, it is best to use this ability. It's damage ratio is comparable to 'Path of Flames' but with a short range. (Can proc. 'Combustion' effect)
Noxious fumes: One of Agni's greatest attributes to the team. This skill is great when it comes to survivability and team synergy. This skill has a large radius, so stunning a team and/or single target is pretty simple. This ability can be triggered through any of your other abilities. Flame Wave, Path of Flames, and Rain Fire, can trigger the stun effect and be a great tool for initiating a team fight or keeping your team alive.
Agni has a higher skill cap than most other gods. I don't say this implying the god is hard to learn to make useful. I think utilizing his stun and path of flames can really decide how you survive, as well as winning or losing a team fight.
Thanks for reading my guide on Agni, and I will be sure to include more information in the future as I learn more about the game myself!