Archfey and Archfiends
These are the greater and lesser Arch-entities of both the Seelie (Summer) and Unseelie (Winter) courts. The Unseelie court is also the Archfiends (a little simplification for Warlocks and Paladins …). Some of these figures are generally aligned with one Court or another (from a human perspective), but may to cross borders and act unpredictably. The Raven Queen, for example, is very mercurial and has been known to give clues and hints to adventurers and rogues. Likewise, Manannan is prayed to by wholly good fishermen for a safe outing. Melikki is prayed to by bounty hunters. Oghma is the patron of all kinds of knowing - even the knowing of evil things. Some scholars argue that Lalli and Tellerva are the same entity. You get the idea. These Archfey and Archfiends are the subject of worship by all hominid races. Elves tend to favor Cernunnos, Dwarves favor Goibhniu, and Orcs tend to favor Morrigan, but these general affinities are not required for PCs.
There are a host of minor fey/fiend lordlings for all the various natural phenomenon - The River King, The Stone Lady, the many flower faeries, hags - all with some minor role to play in the cosmos. All nature of lesser fey may serve these lordlings and sometimes in unpredictable ways. For example, it’s common to see trolls and ogres as pawns for a hag, or shock troops for a minor lord.
Nominally, Tiandra is the Queen of the Seelie and Firosa is the Queen of the Unseelie. There aren’t really clerics to either of them, since they don’t seem to grant spells. Likewise Oran is not directly worshipped, being as he is really the abstract embodiment of the earth, although some suggest that Druids’ power derives from him in some way. Others say that it’s Cernunnos. It is unknown if these primordial beings even get involved in the schemes of their children.
Other, sometimes-worshipped entities not part of the Courts
Detailed Descriptions of Select Archfey/fiends
The Prince of Frost
While generally considered a force of cold, uncaring evil - he is the embodiment of the natural death of winter - the Prince has been also seen as a romantic figure, and is sort of a tragic anti-hero in a lot of Bardic epics. His current role as the patron of winter is a reflection of having had his heart broken by Briganta, and the cycle of rivers and lakes freezing (Briganta in his grasp) and then thawing (she escapes) across the seasons is a common story of his attempts to win the heart of one who doesn’t love him back. There is a prophecy that if he ever finds love again, there will be summer and bounty for all the year.
The Prince does have, as one of his clerical domains, the Light domain. It works as described in the PHB, but it’s a cold, blue, chilling light. Still radiant damage, but the cold burning on your eyes of the sun reflecting off snow and ice.
The Raven Queen
The Old One
This is the primordial hag. She is the ancient, original hag. It’s, in fact, unclear if she stands outside the family of Oran, Tiandra, and Firosa, in the same way that The Whisperer does. Her evil is deep and abiding. In fact, the stories told about her cave in the Shadowfell suggest that it’s the worst place to be a child. Ever.