Shadowmere

A lake in the Blessed Realm, lying at the feet of the immense mountain of Taniquetil. The shadow of that great mountain lay across the lake, from which it took its name ('Shadowmere' being a translation of the Elvish name Luvailin). The Shadowmere lay close enough to the Elves' city of Tirion that the lamps and lanterns of that city shone from reflections in its mirrored surface.

We know almost nothing about the Shadowmere beyond its single mention in Bilbo Baggins' "Song of Eärendil", although Tolkien does provide a little more background (such as the lake's Elvish name) in his unpublished index to The Lord of the Rings. Based on the minimal details we have, it is nonetheless possible to make some simple deductions. For the shadow of Taniquetil to fall over the lake, it must have been situated to the east of the mountain (because the Light of the Trees shone from the west). For the lake to reflect the lights of Tirion, it would also have needed to have fallen within the Pass of Light, the Calacirya that cut through the range of the Pelóri. This pass must presumably have been have been the 'valley sheer' described in Bilbo's song.