PROS

• Lots of tankiness due to P
• Great Scaling
• Area of Effect Damage
• Can be played in a lot of lanes (Top, Mid, Bot, Support)
• Front-Liner with high DPS
• Great Healing with and
• Global Ability for Vision and Jungle Control

CONS

1. Early Mana Issues
2. Immobile (that's why you need Ghost)
3. Hard to push lane early
4. Can be countered that champs who outrange him
5. Low early game damage
6. Can be bursted without Zhonya's Hourglass

For runes I selected:

Precision Tree:

Conqueror for mega healing on extended fights.
Presence of Mind for no mana problems on extended fights.
Legend: Tenacity because that is the only source for Tenacity .
Last Stand will make you do more damage when you are on low hp.


Inspiration Tree:

Perfect Timing will be a free stopwatch for a cheap Zhonya's Hourglass.
Biscuit Delivery will help you with the early game mana problems.

Ravenous Flock P

Enemy champions that die will leave behind a Soul Fragment for 16 seconds, which can be collected by Swain after 1 second. Swain's ravens collect nearby Soul Fragments, as well as with Vision of Empire and Nevermove.

SOUL FRAGMENT: For each stack, Swain gains 12 bonus health. Upon collection, he will heal for 3 / 4 / 5 / 6% (based on level) of his maximum health.

Death's Hand Q

Swain unleashes five bolts of eldritch power in a cone in the target direction that deal magic damage to enemies hit. Subsequent bolts against an enemy deal bonus magic damage.

Vision of Empire W

Swain summons a demonic eye at the target location that grants sight of the area for 2 seconds and explodes after 1.5 seconds, dealing magic damage to enemies within, halved against minions, and slowing them for 2.5 seconds.

Enemy champions hit are also revealed for a duration and have a Soul Fragment collected from them.

Nevermove E

ACTIVE: Swain launches a demonic wave in the target direction that deals magic damage to enemies hit.

At maximum range, the wave homes back to Swain and detonates upon the first enemy hit, dealing magic damage to nearby enemies and rooting and revealing them for 1.5 seconds. Nevermove can be recast while champions hit are rooted.

RECAST: Swain pulls the rooted champions over 290 units and collects a Soul Fragment from each of them. If the targets would be brought past Swain, they are instead knocked back the excess distance.

Demonic Ascension R

ACTIVE: Swain frees the demon within, gaining Demonic Ascension and generating 50 Demonic Energy. Demonic Ascension is maintained with Demonic Energy, which decays by 5 every 0.5 seconds, increased to 7.5 after 5 seconds have elapsed. Swain generates 10 Demonic Energy every 0.5 seconds while draining from at least one enemy champion, and fully restores it to the maximum of 50 upon scoring a champion takedown.

Demonic Ascension: Swain is ghosted and drains the lifeforce of nearby enemies, both dealing magic damage and healing himself each second per target affected. The heal is reduced by 90% against minions and Monster monsters. Nevermove's cooldown is also reduced by 20%.

After a 2 second delay, Swain can cast Demonflare and does so automatically when Demonic Ascension expires.

The targets do not have to be visible to be hit by this ability.

Demonflare R

Swain releases a nova of soulfire that deals magic damage to nearby enemies and slows them by 60%, decaying over 1.5 seconds.

In the early game, try to collect as many Soul Fragments as you can with your W and your E to gain Max Health and become tankier, but beware of your mana problems. Try to seek the enemy jungler with W and get vision of them. Also, you can try to steal the enemy camps to be extra annoying and get a lead to your lane. Your ideal first back option is a Lost Chapter as it gives you a lot of mana which is Swain's main early game problem. Poke the enemy and wait for your jungler to gank you. Before level 6 you are very vurnurable to ganks and you do not output a lot of damage. Do not try to all-in the enemy before 6 unless they are pretty low. Just try to scale and try to get your 3 items ( Liandry's Torment, Zhonya's Hourglass and Rylai's Crystal Scepter) as fast as possible due to the fact that this is your biggest powerspike.
Moving to the mid game, you want to scale so continue csing. If the jungler ganks you try to turn the gank with R and 1v2 them. BUT, try not to use your ult before dragon spawns. As soon as it spawns, use your Teleport and fight to secure the objective. Then farm and find easy targets for you and your team to eliminate. Apply pressure to the side lanes and try to get turrets. Be with your team at least 30 seconds before dragon or baron spawns since dragons got buffed and they are extremely usefull. You always want to fight with your Ultimate ready. As you are fighting, Rylai's Crystal Scepter will make you be close to the enemy so your ult doesn't run out. Whenever you are low, pop Zhonya's Hourglass. You will regenerate a lot of HP thanks to Conqueror and your R(Demonic Ascension) . If the enemy team has, for example, an enemy Lucian fed, stack armor. If they have a Cassiopeia fed stack magic resist.

In the late game, try to get easy enemy picks so you and your team can apply pressure. Slay the Baron Nashor so you and your team can push and get inhibitors. If you think you can win 5v5 fights, pop your Ultimate and fight. Finally, destroy all their base and consequently their Nexus. GGEZ. I hope i helped you.

Liandry's Torment is a must-have first item on Swain. It gives him a lot of mana for the early game as well as ability power and Ability Haste, but the most impressive stat is its max health burn.This burn makes Swain melt tanks and it is a really good synergy with Demonic Ascension.

Zhonya's Hourglass is a mandatory second item because it can change an entire fight. You can be easily be bursted in a fight with Swain but if you Stopwatch, you will be able to regenerate a ton of health while having Demonic Ascension and draining the enemy team. Furthermore, it makes you tankier because of the Armor it gives you.

Your third item is going to be [Rylai's Crystal Scepter]. When you unleash your Demonic Ascension, most enemies will try to leave from the range so your ult runs out of energy and ends. So Rylai's Crystal Scepter will slow them making it harder for them to leave. It also gives [health] and makes [Swain] tankier.

Demonic Embrace will amplify Swain's damage due to its burn and most importantly it's unique passive which gives you ap based on your bonus health. This way Swain can have infinite scaling with his passive that gives him health so it gives him ability power and his items.

Your last item should be Spirit Visage because it gives Health(=ability power), increases your healing from Demonic Acsnsion and Conqueror and makes you tankier.