Hello y'all! This guide is made 100% by me, from my experience (Note that Intro and some Requirements things are from the offical website).Hope you will find it useful.
UPDATED:08.20.2008:Added 2 new tips!

I.Introduction:
In the decisive battle between human and demons hundreds of years ago, Ultimate Pluto, the leader of the demons, who was beaten by the forces of humanity. However, he escaped and swore an oath to eternally seek vengeanc upon those who had disrupted his plans.
Now, the Solar Saint is aware that Ultimate Pluto has come back. The demon currently resides in Dis City, a black mark upon the earth.

II.Requirements/Time/Rewards:
a)Requirements:Level 110 or above.
b)Time: Mon & Wed 8:00pm-9:30pm PST or Tues & Thurs 12:00pm-1:30pm PST
c)Rewards:

• For Sure:Experience
• Gained in the 3rd Stage:Lucky Amulet
• May be gained:NightDevil
• Reward of the quest:SpecialDress

III.How it's done:
You shall go in Ape City before the quest starts so you make sure you'll participate.When the quest starts, speak with SolarSaint: "I am here to drive away the devils" and then click "Yes" to be teleported.
The quest is divided into 4 Stages, each with its own objective.
First Stage:
Objective:Gain 5 SoulStones.

After you are teleported into the Dis City quest special map, your first objective is to gather a number of 5 Soul Stones. Soul Stones are dropped from the monsters in the first stage. You shall start right away to kill either Phatoms/UndeadSoldiers/UndeadSpearmen/Revenants or Eidolons. As fast you advance to the next stage, the more mobs you will find there and less people will be there.
When you found all the SoulStones pass the bridge and you'll find SolarSaint at the end of it. Speak with him and click "I'm ready".You will be then teleported to the second stage.
Note: Only 60 players can advance to the second stage!

Second Stage:
Objective:

• Water Taoists: Kill 600 Monsters
• Trojans:Kill 800 Monsters
• Warriors:Kill 900 Monsters
• Fire Taoists:Kill 1000 Monsters
• Archers:Kill 1300 Monsters

When you enter in this stage, make sure you dont waste your time looking around,just go and kill.While Centicores,and Nagas have low HP, HellTrolls have alot of HP.
I suggest you dont wasting your time killing monsters one by one if your an archer or fire, cuz you will lose alot of time. After you finish your objective the system will anounce you can advance to the next stage.Find SolarSaint and choose one of the flank you want to go in.
Note:Only 30 players can advance to the next stage (15 players per flank)
Screenshoots from 2nd Stage:

Third Stage:
Objective:Kill Wraiths until their number decrease to a certain level.

After you are teleported in one of the flank you choose to go in,you may go to find one of the spawn with Wraiths.There are 12 spawns of Wraiths on the map. You may go and look for one, and help all the others, or join your friends on one of the spawn an help him or wait to be teleported. Note that Wraiths may drop Night Devil skill, which is very rare and precious, but the possibility is very low.

When the objective is achieved, all the players in the flank will be teleported to the final Stage.
Screenshoots from 3rd stage:

Final Stage:
Objective:Kill Ultimate Pluto, gain Dark horn and give it to Solar Saint.

This is a free PK map, this means all the participants can PK freely without gaining any PK Points.
When arriving into the 4th stage, you and all the others participants must go and kill Syrens, very powerfull monsters.After all the Syrens are killed, Ultimate Pluto will emerge. This is the moment when all starts to PK because they want the DarkHorn, so after Ultimate Pluto dies, it'll drop Dark Horn.You may fight to get it or wait for the player which got it to give it to SolarSaint.
Note that after Ultimate Pluto dies, you'll have to wait 5 minutes untill SolarSaint will appear. During this time you may go and find the player who got the Horn, to kill it and get it or just wait to be teleproted back in Ape City!If the person who got the Horn got killed, Dark Horn will definately drop!

If you are the one which got the Horn luckily hide well and make sure noone will kill you.When the system says SolarSaint appeared go and give the DarkHorn to him. Make sure you wont get killed while going on your way. Pkers will be very attentive when you'll came.
After you give the Horn to SolarSaint he will give you a Star Sword, and you can go to Find him in AC after you will be teleported out, to get your reward.

Screenshots from 4th Stage:

IV.Rewards per Stage:

• First Stage: Experience worth of Half an ExpBall
• Second Stage: Experience worth of an ExpBall
• Third Stage: Experience worth of an ExpBall,Lucky Amulet and maybe a Night Devil
• Final Stage: JUST FOR THE WINNER: Experience worth of 5 ExpBalls,StarSword
• Final Reward: A special Dress.

V.What are the special items for?

• Lucky Amulet:You can find Mr. Free in Pheonix City to change Lucky Amulet for a new hairstyle. You can also sell/buy it from other players in Market.
• Night Devil Skill Book is an XP skill book. It is mostly used by trojans to increase their speed of attacking stakes in Training Ground.
• StarSword is the item you get from Solar Saint when you leave the last Stage of the quest. Find Solar Soint in AC to claim your reward on the top of the mountain.

VI.Comments,Tips and Hints for each stage:

• First stage:
  1. We all know the quest is for players above lvl 110,but from my experience not everyone can go in and advance to the final stage. I suggest you enter in the first stage so you can get the free exp but if you dont have very nice gears and your below lvl 115 then please make sure you can deal with the monsters
  2. In the first stage you have to find 5 Soul Stones.All monsters may drop 'em but It's proved that Eidolons have a higher rate of dropping them, so I suggest you killing as many as you can if you want to advance to the second stage.Remember:ALL THE MOBS CAN DROP THEM, NOT ONLY EIDOLONS!
  3. If you manage to get 5 SoulStones in less than 5 minutes, you can enter the 2nd stage, scroll out to Ape City and enter again.
     You will not get an extra 1/2 exp ball for re-entering, but you will get an additional exp ball for getting 5 stones and entering the 2nd stage again.(Remined by Red or Green)
  4. Note that monsters in 1st stage are physical mobs BUT they have a long range (similar to magic mobs) and especially Eidolons.They also move very fast, so pay attention to them, especially if your char is weak. (Suggested by KITSU)
• Second Stage:
  1. Second stage is the hardest stage in my opinion because many players participte in it and because of the high number of mobs that must be killed. However, if you finish the first stage very fast you may find many monsters in the second stage, so this can give you an advantage from all the others.
  2. Due to the archers that usually KS other players, some players may PK others.I recommend you being very careful, and if you KS someone without wanting to annoy him, then leave him alone and find another to kill.You can apologize for what you did. Also Trojans and warriors, please understand archers aswell, they have to kill 2 times more than Trojans and 400 more monsters than Warriors.
  3. HellTrolls have high HP and may take along time to kill them but note that they count as 3 kills in the counter. (Suggested by Weatherwax and confirmed by HadesHell)
  4. Be careful where others go, in which flank. A good choice can give you the victory, a bad choice can make you lose. If most of the people went in the right flank and you go in the left one then you will probably lose the third stage, and you wont be teleported to the final one.
  5. If you die in this stage, but you are heaven blessed, ten you may revive by using the revive here function, and continue killing the monsters, because the counter wont reset even if it shows the blue counter.(Reminded by Bryntone and many others)
• Third Stage:
  1. Nothing much to suggest, just dont KS people and if you entered after Nr.8 I suggest you moving very slow or not moving because you may got disconnected when the system will teleport the winners of the third stage to the final Stage.
• Final Stage:
  1. Be careful you dont get killed before and after the Dark horn is dropped, because this is a free PK map.If you are the one which got the Dark Horn, then find a good spot to hide and watch out for anyone that can come.You have to wait 5 minutes for Solar Saint to appear.
  2. If you get killed when you have DarkHorn, It will definately drop!
  3. After SolarSaint appears, make your find out of the bunch of the ppl waiting for you to finish the quest or to PK you.Try not to get killed.
  4. If your guild mates also got in the last stage, then make them support you and make them defend you just incase someone wants to PK you. Also in my server, my guild gets the horn like everytime, and thats because we all work together!

VII.Some few words in the end...

Thank you for reading this, if you actually did, any suggestions are always welcome, please vote on the poll and tell me your opinion about it aswell.If you want you may rep me too.