They are magically able block 'dragon senses'. Whoever had planned devils assault provided them goods needed fool a dragon but not a mage skilled enough summon cheap spider critters :P. This should be IC hint that devils did not come unplanned here and are literally expecting almost anything your guys are able to pull off.<br/><br/>Also what comes to PaleEnchantress possession fiend/pet, it cannot get out from his/her body because all planar travel and ethereal travel is blocked in the area. You can carry him just fine but in this zone is more than one spell-effect on.<br/><br/>Also the pair assassin devils are not just standard kind either. They got some different gear and feat choices and pair more levels in assassin PrC (if you peeked on spoiler).