Tairnadal

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. Unlike the Aereni, who preserve their dead as deathless, the Tairnadal seek to preserve the legendary elves of the past by keeping their memory alive. They believe that by emulating the deeds of their ancestors, they can serve as an anchor to keep that ancestor tethered to reality, and keep them from fading from Dolurrh.
The Tairnadal are a martial culture, many of their legends are tales of conflict; conflict against the giants of Xen'drik, conflict against the dragons of Argonessen, even conflict against the goblins of Dhakaan. Emulating the deeds of their ancestors requires that the Tairnadal seek to maintain a near constant state of conflict.
Ideally, the Tairnadal would fight against an actual threat, their ancestors fought giants and dragons. However, there isn't always a true threat for them to fight, and so in the absence of this they have created challenging war games. They hunt dangerous beasts and fight each other. The Valeas Tairn believe that it is not the enemy itself that matters, just that they are fighting a threat.
This provides some context for the existence of Valenar. The elves promised the Dhakaani that they would never return to Khorvaire unless invited, and they held true to this. When Cyre invited them to serve as mercenaries, they saw an opportunity to engage in combat against a worthy foe. Even when they seized the land that is now Valenar, and when they break the terms of the Treaty of Thronehold, their behaviour is contextualised by their beliefs. They aren't just being belligerent, they need the conflict or they believe they could lose their greatest souls.
Everything that the Tairnadal do is in service to this goal of emulating their ancestors. Whilst most of their legends are tales of conflict, not all of them are, and if a Tairnadal is chosen by an ancestor who didn't fight, then that Tairnadal won't fight. Similarly, there are Tairnadal who abide by a strict code of honour, and Tairnadal who are relentlessly dishonourable. There is a vast spectrum of patron ancestors, and the Tairnadal will do their level best to emulate the deeds of their patron ancestor. It's unlikely that there are many ancestors who are irredeemably evil, they're revered ancestors for a reason, but there are undoubtedly a few. These are the people who defined who the elves were during their greatest struggles, both violent and otherwise.
There are several groups within the Tairnadal who have different beliefs about how best to emulate the patron ancestors. As previously mentioned, Valeas Tairn believe that the enemy itself is not important, what matters is having a worthy opponent. The Draleus Tairn believe that their ancestors fought dragons, and so should they. The Silaes Tairn believe that the Tairnadal should be attempting to retake Xen'drik.
The Siyal Marrain are an order of Tairnadal druids that are charged with keeping and caring for the Tairnadal animals. These animals are believed by the Tairnadal to be the descendents of their ancestors who assumed animal form in their fight against the giants, and were cursed by their enemies and trapped in the animal forms they had taken. The modern Siyal Marrain preserve the bloodlines of these Tairnadal animals, and help to match elf and animal to ensure that a bond is formed between them. Beyond this, they are warriors just like most of the Tairnadal, and they bring the power of nature to bear against the enemies of the people.