Helcar (also spelled Helkar) was a great tower, raised by the Valar (some traditions say by Melkor) to house the northern of the Two Lamps, Illuin. It was placed in the north of Middle-earth, and together with the southern lamp Ormal, lit up all of Arda.
Helcar was destroyed by Melkor, and collapsed. This ruined a great part of Middle-earth and formed the inland Sea of Helcar.
Other versions of the legendarium
In earlier versions of the legendarium, Helcar was made by Melkor out of ice, a substance not yet known to the Valar. The heat of Illuin then melted the tower, and the meltwater formed the Sea of Helcar.