Arachnos occupy a region of the map that gets changed quite heavily between Vortex and Mortal Empires campaigns, so it is hard to generalize with them. However they and the Karak Zorn Dwarfs are the main occupants of the Southern portion of the Worlds Edge Mountains. Given that the Dawi rarely leave their hold, Arachnos will usually control the majority of the mountains. Terrain that is difficult for most factions to occupy outside of Greenskins and Dwarfs. This makes them an annoying and belligerent thorn in your side if they begin sending armies out at you - however, this also makes them a very safe expansion point for the Knights of Caledor. There are few factions they will be friendly to unless you fight their enemies. As the Tomb Kings, it may be best to be friendly with them than fight them, more effort than they are worth. Generally though, one can safely eradicate this faction.