Phoenixes
Phoenixes can technically be categorized as Immortal and Fey.
They are technically immortal, but it’s a tricky kind of immortal. They age to about their mid twenties and then stop aging and live on and on and on, the only catch is they can die just as easily as any human and when they die their ashes drift off someplace safe and then they’re super-heated and solidify into an egg which hatches and bing bang bong you got yourself a bouncing baby bird brain. The thing about phoenixes and rebirth is that they don’t remember any of their past lives. It’s like hitting the factory reset and starting all over. Because of this phoenixes generally keep lengthy records of their lives either through journals, art, photographs, and many other ways. (The Underworld has systems in place to make sure phoenixes keep their possessions through rebirth but it’s all very complicated and we’re not going to get into that right now.)
They are technically fey because they contain certain magical and supernatural elements within their bodies. Primarily in their feathers. Phoenix feathers have a high concentration of a supernatural element that causes rapid healing and regeneration. They also produce a very mild form of something called “Ichor” in their tears and saliva.
Over many centuries witches found a way to use phoenix feathers to make an extremely potent form of Ichor that they use as kind of a healing potion. It can heal any wound, regrow limbs, and even bring back the recently deceased. Making Ichor, however, is very difficult and considering there are only about 100 phoenixes in existence phoenix feathers are kind of hard to come by. Combined with the difficulty it is to make and rarity of its most basic ingredient, Ichor is the most valuable substance in The Underworld.
One more thing about phoenixes: since they are technically fey they are therefore controlled by certain ancient fey magic laws. It’s different for every creature, but for phoenixes specifically, if you call on them by their true name they’ll hear you and come to you. Also if you give them a command by their true name they have to do whatever you say. As you can see this has the potential to go very wrong and because of this they never speak their true name. Not ever.