The veiled caves

Caves hone to the undead which have a veil which reaches wall to wall and appears as a sort of smoky pale translucent wall. When an undead mob passes through the veil it gains temporary invis and strength and when the player passes through they are given slowness and true sight. True sight: randomly nearby mobs and ore clusters glow for a few seconds. Caves would also have dirty floors with dead plants atop and ned dead varieties of some flowers. Finally caves would have thick fog covering the floor.