So, the first thing that will make you improve with Aphelios is understanding his weapon and weapon order. Since this is an In-Depth Guide, I will explain to do every detail about his weapons with some tips as well as explaining how to fix your weapon order.
- Severum:
Passive:
Basic attacks with Severum are non-projectile and have an uncancellable windup. Severum's attacks heal Aphelios for 2% − 7.1% (based on level) of the post-mitigation damage dealt, increased to 5% − 17.75% (based on level) for attacks from abilities.

Healing from Severum in excess of Aphelios' maximum health is converted into a Hybrid resistances icon shield for an amount of up to 10 − 140 (based on level) (+ 6% maximum health), lingering for up to 30 seconds.

Active:
Aphelios enters an onslaught for 1.75 seconds, gaining 20% (+ 10% per 100 AP) bonus movement speed and automatically performing up to 6 (+ 2 per 100% bonus attack speed) attacks over the duration against the nearest visible enemy, prioritizing enemy champions.

Attacks alternate between Severum and his current off-hand weapon, each dealing 10 − 40 (based on level) (+ 20% − 35% (based on level) bonus AD) physical damage, affected by critical strike modifiers, and applying on-hit damage at 25% effectiveness.
You can't auto-attack, swap weapons or cast ult while this ability is in use.

This weapon is good overall and is the most interactive weapon, there is no bad combo with Severum on the arsenal. The best weapons to pair with are Crescendum and Infernum, since you can get chakrams with the active way quicke rthan any other weapon and you can heal a lot from Infernum active. Always use Severum active if you have less than 10 bullets on the wave or champions since it heals you more than using it with autoattacks. Remember that the active also gives you some movement speed, if you want to get somewhere quicker or escape faster you can use the active. This weapon is not a projectile, so it can trespass any anti-projectile ability like Yasuo's W.

-Calibrum:
Passive:
Aphelios gains 100 bonus attack range while Calibrum is his main weapon. Enemies damaged by Calibrum through an ability are marked for 4.5 seconds, revealing them for the duration. Aphelios' next basic attack against a marked target uses the current off-hand weapon and has 1800 range, increased missile speed, and a brief cast time that lasts shorter based on Aphelios' proximity to the target.

The empowered attack will consume the marks from all targets, dealing 15 (+ 20% bonus AD) bonus physical damage to the main target for each mark consumed. If Calibrum is the current off-hand weapon, the main weapon is used for the attack instead.

Active:
Aphelios fires a bolt of energy in the target direction that deals 60 − 160 (based on level) (+ 42% − 60% (based on level) bonus AD) (+ 100% AP) physical damage to the first enemy hit.

This weapon is great for short range compositions, since the active has crazy range and it pairs well with Infernum or Crescendum. Always rush Calibrum-Cresecndum against all-in compositions, since you can stack chakrams fast with the crescendum active.

-Gravitum:
Passive:
Basic attacks with Gravitum slow enemies by 30% for 2.5 seconds, decaying to 10% after 0.7 seconds.

Active:
Aphelios expunges all enemies with Gravitum's slow debuff, dealing 50 − 110 (based on level) (+ 26% − 35% (based on level) bonus AD) (+ 70% AP) magic damage and rooting them for 1 second.

This weapon is great for kiting with Lethal Tempo, enemies won't be able to catch you. It synergizes with Calibrum and Infernum. This weapon sucks with Crescendum, since it doesn't use the off-hand weapon. Remember that if you want to get rid of this wepaons you can cast the active the momemnt it hits the target, so if you're quick enough you can do auto Q and get rid of it faster.

-Infernum:
Passive:
Basic attacks with Infernum shoot a fire bolt that upon arrival splits into a cone of 4 lesser bolts behind the target, dealing damage to enemies they pass through. The fire bolt deals 110% AD physical damage to the primary target. Secondary targets hit by any bolt are dealt 82.5 / 110% (based on level) AD physical damage, reduced to 25.3 / 33% (based on level) AD against minions.

Critical strikes also apply to damage dealt to secondary targets and instead spray 6 missiles in a 50% wider cone that deals critical damage.

Active:
Aphelios unleashes a wave of energy in a cone in the target direction, dealing 25 − 65 (based on level) (+ 56% − 80% (based on level) bonus AD) (+ 70% AP) physical damage to all enemies hit and locking onto each of them. After 0.25 seconds, Aphelios then fires a volley of attacks at each locked-on target from his current off-hand weapon, dealing 100% AD physical damage and applying on-hit effects at 100% effectiveness. The damage is affected by critical strike modifiers. There is no Range icon range limit for locked-on targets.

This weapon has insane AOE damage, and the active is one if not the best active for Aphelios. This weapon is good with any other weapons, but it's especially good with Calibrum and Gravitum. Your active range is slightly larger than your base attack range.

-Crescendum:
Passive:
Basic attacks with Crescendum hurl the blade at the target, which lingers for 0.25 seconds before homing back to Aphelios. He is unable to declare basic attacks until he retrieves Crescendum, but the attack timer is reset once caught. Whenever Aphelios casts an ability that would require him to throw Crescendum, he instead fires a spectral Chakram at the target that similarly returns to him. Aphelios accumulates the Chakrams he catches, up to 20, which last for 5 seconds or until Crescendum is depleted of Moonlight. Attacks with Crescendum against champions will refresh the duration of Chakrams. The chakrma comes back quicker if it critically strikes, and the damage increases based on critical chance.

Active:
Aphelios deploys a lunar sentry at the target location that arms after 0.35 seconds, lasting for up to 20 seconds, during which it is inactive and untargetable. The sentry activates if an enemy is within range of it, reducing its duration to 4 seconds and becoming targetable. Sentries have 6 health and take 3 damage per ranged basic attack and 4 damage per hit by abilities. Turret attacks destroy sentries instantly.

The sentry grants Sight icon sight of its surroundings and autonomously attacks the nearest visible enemy in range with a replica of Aphelios' current off-hand weapon, dealing 31 − 100 (based on level) (+ 40% − 60% (based on level) bonus AD) (+ 50% AP) physical damage per hit. The sentry can critically strike and benefits from both Aphelios' attack speed and critical strike chance at 100% effectiveness.

This weapon is great for dps, but you need to understand about positioning and kiting, since you need to move closer and closer to your target. It synergyzes with Calibrum and Severum. Use your turret to zone enemies, it can melt tanks with Calibrum.

Aphelios ult:
Aphelios casts forth a lunar spotlight in the target direction that briefly grants sight of the area along its path and stops upon illuminating an enemy champion. Alune smites the area centered on the illuminated target, dealing 125 / 175 / 225 (based on level) (+ 20% bonus AD) (+ 100% AP) physical damage to enemy champions struck and locking-on to each target hit, as well as granting sight of the area for 2 seconds.

After 0.3 seconds of the illumination, attacks based on Aphelios' current main weapon will launch from the sky against each locked-on target, dealing 100% AD physical damage and applying on-hit effects at 100% effectiveness. These attacks can critically strike for (20% + 45% 45%) AD bonus physical damage. There is no range limit for locked-on targets.

Calibrum: Applies an empowered mark that deals 50 / 80 / 110 (based on level) bonus physical damage per mark consumed. Good ult for outranging a single target. Use this to finish off low hp targets at a long range.

Severum: Heals Aphelios for 250 / 350 / 450 (based on level) if at least one enemy champion is hit. Don't be afraid to use your ult on this weapon, staying alive is way more valuable than not having ult for a minute and a half, this ult can be lifesaver for dives.

Gravitum: Increases the initial slow to 99% and empowers Binding Eclipse to root targets affected by the enhanced slow for 1.35 seconds. Good ult if you can chain cc them into another chain cc ult (Orianna, Malphite and Amumu ult).

Infernum: Deals 50 / 100 / 150 (based on level) (+ 25% bonus AD) bonus physical damage on the initial blast. Attacks splash in a 400 radius instead of a cone, dealing「 90% of that damage. Enemy champions will take damage from overlapping areas. If you hit a five man ult, you pretty much won the teamfight.

Crescendum: Generates 5 additional spectral Chakrams that return to Aphelios from the first enemy champion hit, for a total of 6, on top of those from other targets hit. Always try to hit the most amount of targets, you really need the chakrams.

Little tips here:
-You can buffer all of your weapons actives, but you can't buffer your ult.
-Runaan's applies all of your weapons effects but crescendum. If you apply Calibrum mark and you have Runaan's, the off-hand hit will be sent to the other targets as well.
-Calbrim mark, infernum active and your own ult apply on-attack effects like Kraken Slayer passive. Remember it when you wanna hit a mark so it applies Kraken's passive on the right target.
-Try to be efficient with your weapons. You can apply 4 different actives in a fight if you understand how the champion works, and that will make the difference between a good and bad Aphelios.
-Don't be afraid of testing, this champ needs a lot of practice.

Now, let's talk about the weapon queue.

You start with Calibrum and Severum, then the next weapons are Gravitum, Infernum and Crescendum. Whenever you deplete a weapon's ammo, it will go to tthe last place in the queue, for example, when you deplete Severum ammo, you will get your next wepaon (Gravitum) and Severum will go to the last place of the queue, so now your queue is Calibrum > Gravitum > Infernum > Crescendum > Severum. The optimal order for Aphelios first rotation right now is:
Severum > Calibrum > Infernum > Gravitum > Severum > Crescendum. Then, after you finish your first rotation, just use the oldest weapon on your arsenal. For example, once you finish your first rotaiton correctly, you will have Crescendum-Calibrum, since Crescendum is your oldest weapon you will use it and get Infernum, and now you will use your current oldest weapon (Calibrum).
This will allow you to have sick combos like Calibrum-Infernum, Infernum-Gravitum, Calibrum-Crescendum and the well known Severum-Crescendum combo.

Now, about fixing your weapon queue. It's fine if you mess it up, it always happen in situations wher you are forced to deplete a weapon, we're human after all and we make mistakes so don't think you're bad if you mess up your queue since you can fix it. As I explained before, when you deplete the ammo for a weapon, it goes to the last place in the queue, so let's say you messed up and you depleted Calibrum first before Severum, how can you fix this? Deplete weponas in the order you get them on your first rotation to go back to the initial weapon queue. So you will go Severum > Gravitum > Infernum > Crescendum, this will allow you to have back Severum-Calibrum and you have succesfully restarted your weapon order, now you can get the right order by depleting Severum first and follow the order I explained in this text.

Well, first of all I want to make clear I am not the best Aphelios you will ever see, this is a pure theoretical guide, I may not be the best but I've been playing this champ for over a year and I know pretty well most of the stuff he does. This being said, I'd be glad if you have any recommendation for this guide, constructive criticism is always welcome here.

I will try to update this guide based on the patches, since I use this guide myself and some of my friends use it as well.

Also, I want to let you know that all I learned from Aphelios was from this EUW OTP called "Aleksis007", you can find him on YouTube by the same username or clicking on this link, this guy makes good explanations about Aphelios and I really like his content.
Aleksis007 channel.

So, thanks for taking the time to read this on-depth guide, I really appreciate it.(Fan art by Qi Jingyan)