Aereni

One of the things that defines elves in Eberron is their relationship with the past, and specifically with their dead. Elves live for a very long time compared to other races, and this means that the weight of their lives is heavier. They have real trouble letting go of the past, and this is especially true when it comes to people. If you've had someone around for the last seven hundred years, it would be really, really hard to let them go and accept that they weren't around anymore.
Every elven culture is defined by how they choose to deal with this. In Aerenel, they choose not to let go of their greatest souls by preserving them as Deathless, positive energy undead. These deathless form the Undying Court, and they are kept animate by the energy of the Irian manifest zone that surrounds Aerenel, but also the devotion of their descendants. A wizard training in Aerenel doesn't just learn from the writings and ideas of the elf who first invented fireball, they can actually learn from them directly! 
It is believed on Aerenel that the Irian manifest zone can only support a limited number of Deathless, and so they have been very selective about who actually gets to earn a place in the Court. You have to be truly exceptional, a soul that Aereni society simply can't afford to lose, to really be considered to ascend, and this is the driving force behind their lives. Every Aereni spends their lives, which are very, very long, trying to improve themselves so that they can be found worthy of joining the Undying Court. For those who don't quite make the cut, but are still exceptional, bright souls, they can be preserved as Spirit Idols. Unlike a Deathless, which has a body and can move, a Spirit Idol is a soul bound to an object. They can be communicated with, but it's not quite the same.
The key point in understanding the Aereni state of mind is in how they choose to deal with loss, and letting go of the past: they don't. They don't let go of the past at all, their entire society is built around not letting go, they keep their greatest souls with them.
In addition to this, the Aereni have a very negative opinion of Mabaran undead. In a way, they view the Deathless as a form of sustainable undead, as they are animated by the power of Irian, which is itself the power of creation. Mabar is destruction and entropy, and undead that are animated by Mabaran magic are always ravenous and feed on the life force of others to sustain their existence. The Aereni believe that Mabaran undead drain the life force from Eberron just by existing, so even those that don't actively predate others still drain the life from the world. A mummy doesn't need to kill others to live, but their touch still withers flesh. It is seen as the duty of Aereni to eliminate undead where they encounter them, and the Deathguard of Aerenel are an elite force dedicated to this task.