The Lich King on the Frozen Throne
The Frozen Throne was the receptacle of the soul of the Elder Shaman Ner'zhul following his transformation into the dreaded Lich King. It was shattered by Arthas Menethil, and the remnants now serve as the actual physical throne of the new Lich King created from the fusion of the souls of Arthas and Ner'zhul...
History
From the WarCraft III manual:
"When Kil'jaeden cast Ner'zhul's icy cask back into the world of Azeroth, the hardened crystal streaked across the night sky and smashed into the desolate arctic continent of Northrend, burying itself deep within the Icecrown glacier. The frozen crystal, warped and scarred by its violent descent, came to resemble a throne, and Ner'zhul's vengeful spirit soon stirred within it. From the confines of the Frozen Throne, Ner'zhul began to reach out his vast consciousness and touch the minds of Northrend's native inhabitants."
The term "Frozen Throne" literally applies to this crystal, about the size of a tauren in full armor. Kil'jaeden crafted the original (which was massive- probably about the size of a decent castle) to house Ner'zhul's (now the Lich King's) soul. The harsh entry into Azerothian airspace and the impact with the glacier greatly reduced its size.
Illidan Stormrage was charged by Kil'jaeden with destroying it, and to this end recovered the Eye of Sargeras and brought it to Dalaran. Illidan and his Summoners used the Eye, combined with Dalaran's natural magical energies, to send devastating earthquakes to destroy Icecrown and the Frozen Throne. The intervention of his brother Malfurion, his former jailer Maiev Shadowsong, and Prince Kael'thas stopped Illidan from succeeding, and he fled.
Kil'jaeden ordered a second attempt, and Illidan, along with his enemies-turned-servants, the Blood Elves, and Lady Vashj and her Naga, personally invaded Icecrown so that Illidan could smash the Throne in person.
Illidan again failed, and Arthas smashed the Throne- releasing Ner'zhul's spirit and forcing the two to merge into the new Lich King. "Frozen Throne" now most nearly means the remnants of the crystal that Arthas physically sits upon, as an actual throne.