It is the spawn in annimantion for the spiders they used.
If you are creating the object by a script Change the bUseAppearAnimation Option to TRUE.
// Create an object of the specified type at lLocation.
// - nObjectType: OBJECT_TYPE_ITEM, OBJECT_TYPE_CREATURE, OBJECT_TYPE_PLACEABLE,
// OBJECT_TYPE_STORE, OBJECT_TYPE_WAYPOINT
// - sTemplate
// - lLocation
// - bUseAppearAnimation
// - sNewTag - if this string is not empty, it will replace the default tag from the template
object CreateObject(int nObjectType, string sTemplate, location lLocation, int bUseAppearAnimation=FALSE, string sNewTag="")
If you are spawning them from Encounters. You will need to change there default onSpawn script nw_c2_default9.
// * If this is set, the NPC will appear using the "EffectAppear"
// * animation instead of fading in, *IF* SetListeningPatterns()
// * is called below.
// *
//SetSpawnInCondition(NW_FLAG_APPEAR_SPAWN_IN_ANIMATION);
