The Arasai are a race of malicious fairies that delight in keeping down what they see are the lesser races. They have great affinity for magic. They were created by the Dark Elf Queen Cristanos, who tortured members of the Fae race until they reflected corruption. They worship their Queen like many other races worship their gods - and in many ways, it's an accurate comparison. They start in the Darklight Wood near the city of Neriak.
They have two innate abilities and many Racial Traditions, which they can pick one of every 10 levels up to 80. Their innate abilities are constant slowfall and the Wind Walker effect, which increases their running speed by 50% for 36 seconds on a five minute cooldown.
Their very weak strength and stamina makes it difficult for them to be good fighters until they get better geared and higher level. Even then, they'll need to equip themselves differently than a more focused race would. Their high agility makes them good scouts, but their weak strength unfortunately hampers their offensive potential. They make good intelligence or wisdom based casters - particularly those with light armor, as their high agility will help them avoid incoming hits. Their Racial Traditions give the most benefit to casting classes that need to drop a lot of threat, as you'll see below.
Escaping Dive: Area threat drop, 10 meter teleport. Instant cast, 5 minute cooldown.
Aerial Dodging: 2% melee/spell crit, +5 defense
Glide: Jump for a greater distance.
Arasai's Endurance: In-combat Power Regen - 9 per second at level 80.
Arasai's Flight: +5% movement speed.
Magic Protection: +3% magic resistance.
Arasai Savvy: +5 intelligence.
Neriak Precision: +5 artificing.
Transmograpy: +5 transmuting.
Dark Envy: +5% melee/spell crit, 10% casting time reduction, +25 damage per second, parry and defense reduction. 24 second duration, 5 minute cooldown.
Their traditions show a clear affinity for arcane casting, and if you want to get the most out of your Arasai it would be the best class choice to go with it. The +5 defense increase is actually a fairly strong statistic for tanks, and compares well with similar tanking traditions given by other races. Their lack of stamina hurts, but the additional agility is actually quite useful, particularly for avoidance tanks like Bruisers. Their abilities are quite similar to those of the Fae, so if you'd like to Betray, it might make more sense just to create one of those instead.