What is Sidarhael?
Firstly, Sidarhael (meaning Throne in the ancient elven tongue) is the name of an original D&D campaign setting created by professional Game Master and Filmmaker, Russell Kirkby as a love letter to his wife, author Helen Kirkby (The Brazen Serpent, available at Nephilim Press).
Sidarhael has since grown into the setting for the show, The Vaingate Chronicles--a roleplaying focused and narrative-based weekly D&D Livestream featuring a group of dedicated performers and actors who can be found on Twitch on Friday nights at 7 pm (+12 UTC). (Helen does the music and sound design for this when she has time too!)
Also, you can personally venture into the rich tapestry of the myth, magic and geopolitics of Sidarhael by booking a campaign with Russell Kirkby via Questbook. Right now Sidarhael hosts several different groups of Heroes acting independently of one another--besides the Servants of the Fox in the Vaingate Chronicles--most famously, The Frostmoor Crows and the heroes of Uthdark’s Call. Two other roleplaying groups who have elected to make their game sessions available to the public too.
Thus, Sidarhael is the setting of multiple D&D campaigns--past and present--which serve to inform the world concurrently and may overlap from time to time.
If you are interested in knowing more about the world itself and what makes it magical and unique as a roleplaying experience, see this article about Mythic Roleplaying in Sidarhael by its creator.