In Uqua, the Ocean God Chantry: Part of the Uqua raid expedition; these can appear any time a player dies (see this quest entry for raid details).
In Oceangreen Village: Part of the anniversary task "Music for a Weary Soul" (see this quest entry for information).
In Shard's Landing: Part of the task "Sleepless Nights" (see this quest entry for information).
In the Ethernere Tainted West Karana: "a vengeful spirit" (click for map image) has a chance to spawn after the fifth step is completed in the heroic adventure "Life After Death". It hits for a reported max ~16,000 and has ~5.5 million hitpoints.