It's a very nice sword for Assassins and maybe dual-wielding Gladiators? Ok, so here's how YOU go about getting your very own Inferno sword. Aionology covers the first part of the quest quite nicely: Aion Quest Help – Fire Sword (Elyos Only)
Step 1: Head to North Eiron Desert and kill Dried Gnarls and Mottled Basilisks until it drops. This might take awhile. A loooong while. It's not exactly a common drop. If you have a bit of cash, and hate looking at the idea of potentially grinding for hours, you might consider just buying the thing. It runs from 50,0000 on up.
Step 2: Take your shiney new fire sword and run up to Stataloca's Grave and find Garparos. He's on a little hill to the south-west of the priests. My assassin is looking SW in this pic, you'll see G on the far hill.
Step 3: Get the quest to collect the bits from the nearby fire spirits, do that. and bring the bits back to Garparos.
Step 4: He presents you with the shiney green fire sword.
Now you have to level to 32, if you're not already there.
You're Level 32, Off to Sanctum
Go to Sanctum and speak to Regmos. He's on the platform to the right of the Airship dock. He'll give you the quest and send you on your way. The quest has you collect some bits from flaming Barghests and Magma Spirits, as well as a feather from Griffonix.
Note: unequip your fire sword before you get, or hand in, the quest.
Note: You can solo the entire quest, I did, but it isn't easy and you should carry some nice life serums or potions. Or... you can take a friend or two.
Now you're back to Eltnen and you'll fly off to Agairon village and then run to Geroch Crater. You'll go deep inside to find Magma Spirits and Burning Barghests. Both are level 37.
Kill them until you have all the parts. They're pretty easy (for something that's +5 levels) and seem to have a 100% drop rate for those parts. The crater is located in southern Eltnen, southwest of the Agairon village (and not in the image, below.) By the way, this might be a killer grinding spot in a few levels.
Head north of the village to the Drake Habitat and pick up the two "Kill Vigaro" quests. I don't know if you need to have done the previous quests there to get those. You'll be killing them anyway, so it's more almost free XP
Head to the east side of Kuriullu mountain and look for Griffonix. If you do it in the daytime he's friendly and you can talk to him and get the feather quest. You can also grab a second quest (which is an escort for his baby) so get that one, too. It's almost free XP.
If you see him at night he'll be hostile. Wait for daylight or grab a couple of friends and kill him. (There's more XP if you chat with him, but what the heck.) I assume that if you kill him you'll get the feather, I waited until morning and talked to him.
He'll have you gather fur from Pretor looters, Vigaro hearts, and "Spirit of Wind." The looters come out at night, but there's a pod of four that spawn at the top of a massive rock stair that leads down into the chasm, marked with the red X.
Head down the main road, south, for Vigaros and spirits (my marker is at the top of the road. ) Kill any of the spirits in the area for the "spirit of wind," the Vigaros are a bit south of the spirits.
The looters, spirits, and Vigaro are all about 32-34.
The baby bird (you did get the escort quest, right?) is off to the left, as run down the road, at the northern end of the Vigaro area, inside one of the Vigaro TeePee things. You don't have to excort him all the way back, just to the bridge at the north end of the area.
Once you have all your bits go back to Griffonix and hand them in, along with the escort. Run back to the Drake habitat and hand those in.
Go back to Sanctum, find Regmos, and hand it in. Make sure the sword is not equipped when you hand it in. You'll get your shiney new sword just waiting for you to slot in three shiney new crit stones.
