Invasion of the Sload. More information on Sload, and the lycanthrope variations can be found here: https://www.youtube.com/watch?v=XHirn93DlfI

Sload are disgusting slug-like humanoid creatures that are experts of necromancy. They are not just disorganized beasts though; they were once the most significant threat to all races of Tamriel in the first era. They laid waste to the high elf stronghold of Errinorne, seiged Skywatched and created the Thrassian Plague that devastated the population of Tamriel.

The "All Navy Fleet", the largest fleet in the history of Tamriel, set sail to destroy the Sload Empire and was successful but the Sload are far from gone and still seen in the fourth era (ES: Skyrim timeline).

A few interesting changes that would be included in such a content pack would be a Necromancer Class to be added, new mechanics to be added such as underwater enviroments, cities to be changed and destoryed and possibly the first new race to be added since ES: III.

Quest Start: Begin in a coastal city with what seems to be peace time work - Investigating a murder, hunting down cultists, chasing a thief, putting an end to a skooma operation, or anything will work. During the quest, you are distracted by survivors fleeing from the docks and reports that a ship full of undead had docked and attacked the city.

Going to the docks, you discover a large invasion of undead coming from the ship and rising from the sea itself. On the horizon there are strange ships anchored far from the shore. You try to investigate the source of the undead and try to fight them back but each victory you cause, is several more defeats across the city. One victory gained the glimpse of a creature you have never seen before involved in a necromancer ritual on the docks. You are unable to stop the ritual though and repelled by the undead. Eventually the city is lost, and you are forced to retreat to report to the faction leader of what occurred.

Any road near the shore is met with new undead, but the deeper inland you travel toward the faction's capital, the less undead you encounter. Upon reaching the capital city, you find it crowded with refugees and more tales of reports that every coastal settlement has been attacked by the undead, or at least the vast majority of them.

Informing the faction leader of what occurred, they suspect the Sload which they have only heard the stories of (except maybe Aryenn since she disappeared for a while and is quite experienced). Several missions across the zones would require trying to gather information on the Sload, and trying to halt them wherever you can.

During a naval voyage, your ship spots a battle already in progress out at sea. Depending on location, it is either the Maomer, or Lilmothiit combating the Sload. In an effort to gain allies, you aid and board the Sload flagship and kill the Sload admiral. The other races refuse to join your alliance, but agree to aid in the fight against the Sload.

Battles progress in this long war across the sea and land. With two new races, the battles are still a struggle as cities fall, are retaken, and fall again. I'd like to see some cities devastated by this war, guild halls burned (that will add to the era's legacy of having few records survive) and use of siege that we have in Cyrodil on PVE cities. Maybe even have real time updates of assaults in progress, and make them normal trial difficulty so that it'll feel like a real battle and add some much desired end game content for players without being instanced.

This expansion would have no end, until the next expansion (so a couple years) with a trial invasion on Thras would put an end to it (similar to WoW: Lich King expansion).