Ark is a fighter that can also act as a mage. His niche is to be an antimage with great initiation capability but with slightly less burst and more sustained damage. Here are a few explanation about abilities:
Passive: A silence and magic damage on AA every 12 seconds. Can cleave in an AOE (Imagine Diana's cleave) when comboed with e (tracking beam). Can be used to initiate fights with a huge silence on the enemy AP carry, making it good in 5v5s.
Q: This ability defines why he can go AD. It has a high manacost and high CD, so it is bad for spamming early game, but the Bonus AD ratio is Khazix level but can only be used every 8 seconds.
The Q can be chained with his E, tracking beam to suppress a target enemy champion. Suppress on a normal skill is not that broken. It is a stun where you also "stun" yourself, so it is worse than a stun. The % damage is an execute, and to make sure even if you go AP there is SOME damage on the Q, therefore Q can be optimally used for both AD and AP builds.
The suppress should be used in 1v1 skirmishes, as no one can harm you while you are suppressing the opponent.
W: Low manacost shield, bad initial value, but can be VERY useful if no one can pop it. Should be used earlygame to trade with w > aa.
Gives MS if the shield is not popped. Allows him to move around the map a little more (2 second speed up is not very good) as well as chase, but if he is chasing someone the shield value will be low, and can be burst down quickly, and so the shield is not as useful. Bad for running away as it can be popped, but can be used to save your life.
This skill is better with AP as you get higher initial shield values, but if you can extend fights AD will definitely be better.
E: High manacost and CD at lower levels but good AP ratio. Can be used for waveclear which is why if an AP build is adopted this skill should be maxed. The 1.25 sec delay and 325 AOE are exactly the same as brand's pillar of flame.
The tradeoff is that if you use it for waveclear you cannot e>q or e>aa or e>r an enemy champion.
R: The channel time is to choreograph an initiate for your enemy. The projectile is slow at around fizz ult missile speed. The ultimate can follow a tracked champion by targeting them with the cursor,but can also be blocked by a champion, like caitlyn's ult can be blocked. There is a small aoe, that if you manage to hit more than one champion, your teamfight is much easier to win.
Opponents have to make a compelling choice if they are hit. Tank out the damage, which is extremely high if an AP build is used, or use a spell and slow yourself massively, which allows yourself to be caught out even more.
Players have to note manacosts or CDs. If a champ spams all their spells and gets hit by this the base damage is insanely high.
By using e>r, you can ensure a hit, but you lose your chance to gapclose.
This champion's kit is designed to work with AP or AD or even a weird hybrid build.
AD builds make use of Q as damage and slow, W to fight extendedly with others, E to initiate and R to hopefully slow someone down massively.
AP builds use more passive for burst, W for instant shield, E for waveclear and initiate and R for hopefully massive damage.