The Sacred Grove, usually referred to as simply the Grove, is located in Companion's Field on the Palace grounds in Haven.
The grove is where King Valdemar retreated to pray for a miracle, that resulted in the Companions. The original Companions all emerged from the Grove as adults. Once the population was large enough to become self-sustaining, the only Companions to be Grove-Born were the stallions who Chose the Monarch's Own Herald, and the mare Gwena.
During the time of Herald-Mage Vanyel, the Grove had a temple which was the site of Tylendel's funeral. It was also used decades later for the wedding of Prince Sedric and Lydia. By the time of Herald Talia, the temple is long gone, but the temple's belltower remains. It is the tower that houses the Death Bell.
Companions are noted to generally avoid the Grove unless the Death Bell is ringing. (Magic's Pawn)