Figured I'd write something up whilst I'm not doing anything. Here is what I came up with...
The forests of the world are many and vast, only a small majority of these are sentient however like Athel Loren. Asrai treasure the life of all these forests more than the rest as ancient and pure. Some forests are so corrupted now by man, greenskin, beastmen and the other countless denizens of the world that it would be futile for the Asrai to waste resources trying to protect them all.
The Asrai also realise that for the likes of man to evolve and wage war upon the forces of chaos they need the resources forests provide. The sentient forests of the world however are too precious and so are protected fiercely.
It is therefore left to a few elves to travel the many ancient pathways linking the forests of the world to reach these forests in times of danger to them. It would be foolish to leave forces there permanently for they would leave the Asrai numbers few as a people and Athel Loren is the most precious of all these forests. So it is left to the spirits of these woods to defend themselves when attacked until a message is sent to Athel Loren for aid.
This may seem foolish as a message travelling across the world would take forever to reach its destination even on horseback. The secret ways of the forest however can be travelled in a single day by one skilled in their paths. So it is that the forest walkers when called will go wherever in the world they are needed without hesitation or thought of personal safety regardless of the numbers against them or the threats they may face.
These few arrayed with the spirits of the Forest they go to will form a coherent fighting force and make the attackers rue the day they set eyes upon that which they desire. Then after healing any damage done with magic or other skills they possess the Asrai melt away, ready for the next threat.
The shadow sentinels are a band of forest walkers with a rich history. Many a foe has fell with a black shafted arrow or been skewered upon a spear point by these Asrai.
They traditionally wear black and have dark blue skin. That is not to say they actually have blue skin but it appears that way to anyone who has come across them. The shadow sentinels long ago learned that where they go they could end up in a fierce blizzard, a raging storm or under the merciless heat of the sun. Knowing this and their knowledge of the plants of the world they mixed the sap of a tree that grows deep in the heart of Lustria with the flowers found around the wild wood in Athel Loren and a certain arachnid found only in the Haunted forest. This combination makes a blue paste the Asrai apply to the whole of their naked bodies and it protects them from the heat of the sun, the chill of the wind and the bite of many exotic bugs. It can be peeled off like a snake shedding its skin though the Shadow sentinels are very rarely seen without their blue hue.
They wear black traditionally because no matter the season or time of day there are always shadows to hide in and launch ambushes from. It is here that the Shadow sentinels take their name though to their enemies they are just blue daemons come to sow death amongst them.