Obtain 5 Infused Saronite Bars and the Forgemaster's Hammer and use them to make the Reforged Quel'Delar.
We cannot allow Quel'Delar to remain broken and disgraced. Take the remnants of the sword, along with the battered hilt you discovered, and venture into the Pit of Saron within Icecrown Citadel.
In the pit, the Lich King uses slaves to mine, process, and transform saronite ore. This infused saronite is what we will use to rebuild Quel'Delar.
You must gather bars of infused saronite and capture the forgemaster's hammer, then bring them to the forgemaster's anvil and reforge the sword.