In the Dungeons & Dragons fantasy role-playing game, the phoenix is a type of magical beast. As many toher creatures in the game, it is based on Greek mythology, where a phoenix is a long-lived bird that cyclically regenerates or is otherwise born again. Associated with the Sun, a phoenix obtains new life by arising from the ashes of its predecessor. According to some sources, the phoenix dies in a show of flames and combustion, although there are other sources that claim that the legendary bird dies and simply decomposes before being born again. Phoenixes have proved an enduring allegorical symbol, symbolizing rebirth, renewal or uniqueness and often appearing in modern popular culture. Its first apparition in D&D was in Supplement IV: Gods, Demi-gods & Heroes, in 1976. Notably, it was missing from any Monster Manual for 4e.
The phoenix is an intelligent magical creature (in 3e, original to the plane of Elysium) that prefers places of beauty and sylvan peace. A phoenix can attack with its long, sharp, gem-like beak or its diamond-hard talons, but can also voice a piercing whistle that causes other creatures to pause, and has numerous magical abilities. A phoenix appears similar to a very large peacock with a longer beak in proportion, with plumage of bright violet, scarlet, crimson, and flaming orange colors. The beak and claws of a phoenix are of blue-violet, and its eyes are a deep, glowing ruby color.
There is no particular information for phoenixes in Eberron, but its creator, Keith Baker, designed a different RPG with that name, and he has an article about how to implement it into the Eberron setting. You can find it in his website.
In your campaign
Given its allegorical meaning to rebirth, I am fond of using a phoenix as a custodian for particular items or artifacts in my adventures (“artifact” meaning here “powerful and legendary artifact locked by an entity who doesn’t want anyone else to possess”). The creatures will guard the item with its life, and just killing the guardian may not release the item in question: it must be persuaded by other means. In my case, it was by taking another phoenix in the area, which make both fly away. Of course, hints about that were given previously in the campaign through research.