Luvailin or Shadowmere was a mere in Eldamar under the shadow of Oiolossë.
Shadowmere is mentioned in Bilbo's song of Eärendil in Rivendell:
where pale as glass
beneath the Hill of Ilmarin
a-glimmer in a valley sheer
the lamplit towers of Tirion
are mirrored on the Shadowmere.
As the mere reflected the lights of the city of Tirion, "it must have lain in or near to" the valley of Calacirya.