A Splorgath Nightmare finally spawned for me at the following loc: -661.19, 22.27, -77.11.
As far as I know, splorgath's only spawn in the pool where all of them spawn, until the Nightmare decides to spawn, although it is possible that killing other random mobs in the water might spawn the nightmare--I am uncertain in this regard. Killing spiders, witherans, and snakes might spawn the Nightmare, but he definitely spawned to the south of the undead area at the loc above when he did spawn.
besides the drops where does he spawn.....i've camped the pool where the splorgath are but have yet to see the nihgtmare....is something else or somewhere else his spawn area?
Also Drops: Nargil Sharpening Stone MAGIC ITEM LORE ITEM NO TRADE AUGMENTATION Augmentation type: 7 Slot: HEAD FACE EAR NECK SHOULDERS ARMS BACK WRIST RANGE HANDS PRIMARY SECONDARY FINGER CHEST LEGS FEET WAIST Attack: +10 Recommended level of 58. Required level of 51. WT: 0.0 Size: TINY Class: ALL Race: ALL