IMO, It something along the lines of the Atiesh quest line. You kill/defeat/whatever Arthas, and Frostmourne shatters,the explosion is so big it sends shards everywhere, you take the hilt back to Tirion. Who then sends you all over every continent, and every raid dungeon to fight a special boss that appears if you have the hilt (phasing ftw). After getting all the pieces, you have to smelt a special ore, something like elementium from BWL. After all that, you take it back to Light's Hope, where it is consecrated on holy ground, giving you a redeemed/purified Frostmourne.