The Sidhe & the Sith

The Sidhe & the Sith are a group of fairies (pronounced 'Shee', and sometimes spelt Sighe), related to the Fey with the former being Irish in origin, and the latter being Scottish. They can be good, evil, or anywhere between these two polar opposites. Many are tricksters, rogues, and disturbing killers. A few are well-known and found in many lands once, or still, populated by the Celts. The two are so intertwined that the Sidhe / Sith are part of the standard Celtic folklore & religion. Many of them are also mixed with the dead & undead, such as the Banshee, that the latter seems to be the best way of tying these creatures in with the Celts belief in them.
Most of the Sidhe / Sith pay deference to one of the two councils, the Seelie & Unseelie Councils. Many also have links to the Dark Fey, and all can travel between the Material Plane & the Feywild. Many such as the Banshee and the Kelpie can travel through or deep into the plane of Shadowfell too. In a good few number of cases, the Fey that make up the Sidhe / Sith also seem to have links to natural features in old Celtic lands, such as specific lochs / loughs, rivers & streams, forests, mountains, hills, and mounds. They also appear to be some of the oldest Fey in existence, having been in some of these features for millennia or longer. The exact age of some Sidhe/Sith Fey are unknown, and possibly even unknown to the Fey themselves.
It seems their influence has waned somewhat over the past few hundred years, first with the destruction of many of the old Celtic tribes in continental Europe. This was followed by the introduction of Christianity in many regions, and different human tribes with beliefs in different Fey related creatures. There is still many areas dotted around continental Europe where they are talked about in hushed tones, but it is the British Isles where they still have a firm foothold, at least for the time being.