1) This infinitely repeatable quest can be picked up at level 17, but for solo players I would recommend not to start below level 18 and to bring a top no-grade weapon as well as regular Healing Potions (can only be bought on the mainland). Lesser Healing Potions might not be enough.
2) Erinu asks you to go to Nornil's Cave in the eastern part of the island and kill Shady Muertos. As with many Kamael quests, the names of the monsters he gives you are somewhat different from the mobs you will encounter in the cave. Especially misleading is the term "Guard Captain" on the quest screen. There are highly aggressive lvl 16 Shady Muertos Guards, crossbow shooters with a fantastic aggro range and respawn rate, that do NOT give the quest items. And - further into the cave - there are lvl 17 Shady Muertos Captains that DO give the quest items.
3) Before you reach the required monsters in the deeper part of the cave/subterranean temple, you have to fight your way through the above mentioned Guards and equally aggressive Grotto Leopards, both double HP with strong P.Atk. Be fast, don't hesitate, kill them as soon as you see them. They are only in the upper corridors. When you have reached the area with the iconic statues of past, present and future, you are safe and can concentrate on your work.
4) All Shady Muertos are double HP, but if you are quick about it you will be able to collect the required 80 Feathers in one hour of newbie buffs. The quest item acquisition rate is about 1 for every 2 Muertos killed.
5) Return to Trader Erinu. He offers you 4160a for 80 Feathers, i.e. 52a per Feather. Considering that you will have used up around 1500 Soulshots, 25 Lesser Healing Potions and 10 regular Healing Potions, that is definitely not enough. But there is no room for bargaining, so accept his offer.
This quest is only worth doing if you want to level your character fast. In the one hour it took me to collect 80 Feathers I gained one level. Double HP monsters!
Here is a map of Nornil's Cave.