Morrowind:Disguise

Quick Walkthrough
# Speak to Nileno Dorvayn about business and say that again; Nileno will task you with intercepting orders for a dead Redoran.
# Nileno will give you a helmet to disguise your face when you approach Neminda.
# Find Neminda in the Redoran Council Hall under-Skar in Ald'ruhn. Wear the helmet when you use the code words Orphan of Arnesia.
# Retrieve the orders, and return them to Nileno for your reward of 500 gold and the helmet she gave you.

Detailed Walkthrough
First Order of Business
For your first task as a House Hlaalu member, Nileno Dorvayn at the Balmora council manor instructs you to intercept the orders of a dead Redoran from Neminda in the Council Hall under-Skar in Ald'ruhn. Conveniently, your voice seems to match that of the Redoran. If you are male, the dead Redoran's name is Felsen Sethandus; if female, the dead Redoran is Relmerea Saram. Nileno informs you that the dead Redoran was disfigured in battle, and therefore always wore their Redoran helm in public.

Intercept Orders
Nileno will give a Native Gah-Julan Bonemold Helm, similar to those commonly worn by House Redoran, and you are to wear it at all times when around Neminda. Note that while it may look just like any other bonemold helm, even in name, Neminda can tell the difference somehow, so you must use the specific helm you're given. No other bonemold helm will do. Travel to the Redoran Council Hall under-Skar in Ald'ruhn, and locate the Redoran quest-giver Neminda, who should be just inside the front door. The two quickest pathways to Ald'ruhn are through the Mages Guild Guild Guide, or the Silt Strider. Wear the helmet when chatting with Neminda. Once you meet Neminda, you are to use the code phrase Orphan of Arnesia to identify yourself and collect the parcel. Neminda seems none the wiser, and gives the parcel to you for delivery.

Completing the Quest
Though Neminda tells you to take the parcel to Maar Gan, you are to instead return it to Nileno in Balmora. She will congratulate you and reward you with 500 gold. She says you can keep the helmet she gave you too.

Notes
If you are an Argonian or a Khajiit, you will not receive this quest as you will not be able to wear the full faced helmet. Oddly enough, the helm is all you need to wear. You could go completely naked except for the helm, and even if you're obviously not a Dark Elf, Neminda won't figure out that you're not who you say you are.
If you come to Neminda afterwards, and discuss Orphan of Arnesia, she will tell you to leave and never come back, and her disposition drops 20 points.
Arnesia is a location in eastern Black Marsh. The "Orphan of Arnesia" is a reference to the Arnesian War.
Neminda's orders are in code, but it's a relatively simple reversed alphabet cypher. See Neminda's Orders for the text and translation.
The helm looks exactly like any other Native Gah-Julan Bonemold Helm, so be careful that you don't lose it, sell it, or enchant it.
A subtle hint for which helm is correct, if you lose track, is that the correct helm is worth slightly more than the otherwise identical helms (167 gold instead of 165).
If you ask Neminda about Orphan of Arnesia without wearing the helmet but carrying it in your inventory, she will attack you.
If you kill Neminda, Nileno Dorvayn is disgusted and her disposition towards you drops by 30.
If you ask Neminda about "duties," she will recognize that you are not in House Redoran, but you can still get the orders with the code phrase.
This quest is depicted in the Legends card Imposter's Mission. The Imposter it creates parodies the oddity mentioned at the top.

Quest Stages