The difference between the two is borderline comical. The Aldmeri Dominion is treated to a sunny tropical paradise, while the Ebonheart Pact fights its way through the frozen mountains of Skyrim. On top of that, they seem to be designed almost completely independently from one another.
The Ebonheart Pact
If you choose to play a Nord, Dunmer, or Argonian character, you'll be deposited on a lovely snow-covered island just off the coast of Skyrim with the optimistic name of Bleakrock Isle. You're met by Captain Rana of the Ebonheart Pact, who needs help warning Bleakrock Isle's inhabitants of an imminent invasion. And since everyone in ESO's universe seems perfectly OK with trusting some half-naked guy with a sword who showed up out of nowhere to carry out important duties, you're tasked with venturing into the wilds in search of some missing members of her team.
You're given the choice of three locations to explore in your hunt: Skyshroud Barrow, Orkey's Hollow, and Hozzin's Folly. Skyshroud Barrow, like every other burial ground in Tamriel, seems to be overrun with the undead, while Hozzin's Folly is an abandoned mine that has recently come down with a nasty infestation of bandits. Orkey's Hollow, meanwhile, is just some big ice cavern that locals think "might be haunted," which means that it's absolutely, definitely haunted.
The order in which you investigate these charming locales doesn't matter, but you'll be making a trip to all three before you're through here. In addition to these "main" questlines and a number of smaller sidequests, Bleakrock Isle has an overarching objective that asks you to find (and in many cases, rescue) missing villagers so that they can be evacuated before the invasion hits.
It's completely optional, and at just about any time you can tell Rana to forget those suckers and run for the hills if you're in a hurry to get to the mainland. You do get an achievement for finding them all, however, in case the simple guilt of leaving a bunch of innocents to die a grisly death at the hands of unknown invaders isn't motivation enough, you monster. Plus, the sidequests involved in saving all the villagers reward you with precious experience and loot, so there's also that.
I'll leave the ending bits out of here in order to avoid spoilers, but I can say that I didn't end up feeling that there was much of a climax to spoil. Yes, the identity of the invaders is something of a mystery for much of the questline, but the reveal didn't strike me as dramatic or unexpected enough to warrant the secrecy. But then maybe I just wasn't paying close enough attention. Either way, the story didn't particularly grip me.
Environmentally, I think a lot of players are going to gravitate toward Bleakrock Isle (and the Ebonheart Pact as a whole) because hey, it's Skyrim, and I hear that's been a big thing recently. And it really does evoke the atmosphere of the Skyrim we saw in the fifth Elder Scrolls installment. You have the same desolate, snowy mountain ranges and majestic valleys and Nordic architecture galore. You can even still filch things from those neat-looking little urns. You know the ones.
Ultimately, I think Bleakrock Isle was designed uniquely from its Aldmeri Dominion and Daggerfall Covenant counterparts in that the quest flow seems to be much less linear and clear-cut, largely because of the way the optional save-the-villagers objective has players running all around the isle in search of the sods, which makes it feel more geared toward players with an exploration-focused mindset. That's not to say it's entirely directionless or anything of the sort, but it did strike me as appealing to those who prefer to wander about and see what they find.
As a member of the Aldmeri Dominion, your character is transported from the realm of Molag Bal to an island known as Khenarthi's Roost, which lies just off the coast of Valenwood and Elsweyr, the provincial homes of the Bosmer and Khajiit, respectively. It's not long before players, who begin in the settlement of Eagle's Roost, are approached by a remarkably shady Khajiit by the name of Razum-dar, who promptly suckers them into the fold of Aldmeri Dominion affairs in the area, which include (but are by no means limited to) ruins teeming with the walking dead and a massive hurricane that caused the destruction of much of the Dominion's naval presence on the island.
Of course, it's up to your character to get to the bottom of each of these affairs (while also finding time to run errands for the island's inhabitants), which eventually leads to the "jewel of the Southern sea," the free port of Mistral, where the Silvenar (the closest thing the Bosmer have to a true leader) is acting as an ambassador for the Aldmeri Dominion in an effort to negotiate Mistral's admission into said Dominion.
That's a bit easier said than done, however, thanks to Mistral's longstanding treaty with the Maormer, a race of piratical Sea Elves with a long-standing hatred of their Altmer brethren (and by extension, the Aldmeri Dominion as a whole). Oh, and did I mention that the Maormer are known for their powerful weather magic? They may or may not have something to do with that little hurricane debacle I mentioned earlier.
The Silvenar requests your help in sorting out the negotiations and, being the warrior/assassin/diplomat/errand-boy extraordinaire that you are, you step in on the Dominion's behalf. Without spoiling too much, I can say that events (naturally) don't go exactly as planned; you're asked to add spy, judge, and jury to your list of marketable skills; and matters escalate until it's not only the fate of Mistral but the fate of the entire island of Khenarthi's Roost at stake.
As far as the starter zone storylines are concerned, I enjoyed this one the most, hands-down. Not only are players asked to make some true shades-of-gray moral choices, but the intrigue as a whole is much more compelling than the find-the-missing-people scavenger hunts that comprise the bulk of the Ebonheart Pact and Daggerfall Covenant opening questlines.
Not only is the storyline more gripping, but I found quest flow on Khenarthi's Roost to be considerably smoother than it is on the other two, which often have you running helter-skelter around the zones in a series of somewhat disjointed quests. That doesn't mean that exploration isn't rewarded, however, as there are plenty of out-of-the-way points of interest (not to mention the hidden skyshards) for more adventurous players to discover.
On top of all that, Khenarthi's Roost is aesthetically gorgeous. It's a beautiful tropical island covered with lush tropical forests, sandy white beaches with Bahama-blue water, exotic (and murderous) wildlife, and ruins of lost civilizations teeming with the ravenous undead. While that last bit might put a bit of a damper on the island's tourism campaign, I was particularly taken by said undead-inhabited ruins. There's something about mysterious, ages-old temples overgrown with moss and vines that particularly attracts me, and Khenarthi's Roost was my personal favorite place to begin my adventures in Tamriel.
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