1st Legion

The 1st Legion (or the Expeditionary Forces) was the name of the Alliance military forces who accompanied Prince Arthas in his quest to Northrend to destroy the nathrezim Mal'Ganis. They were ultimately betrayed and slain by Arthas before being raised as undead. Some of their ghosts are still haunting their original landing zone at the Forgotten Shore and the souls of yet more members are trapped in Frostmourne.

Third War
The Expeditionary Forces came to Northrend under the command of Prince Arthas in order to combat the Undead Scourge and slay the dreadlord Mal'Ganis. Soon after arriving in Northrend they met up with Muradin Bronzebeard and his expedition and the forces united with one another. In time an Emissary arrived from King Terenas who ordered the expedition to return to Lordaeron, after Lord Uther had persuaded him to do so. Captain Luc Valonforth, in order to avoid the undead, order the expedition forces to cut their way through the woods in order to reach their ships. As the men began chopping their way through the words Captain Valonforth enlightened the Prince to their new orders. Knowing that he needed his men in order to slay Mal'Ganis, Arthas and a reluctant Muradin ventured forth to burn the ships in order to keep the men by his side. By the time the expeditionary forces finishing cutting through the forest, the last of their ships were burning and with Prince Arthas before them. Arthas quickly blamed the mercenaries (who he had hired) for burning the ships and led the expeditionary forces against them himself.

After that, the First Legion accompanied Arthas to defeat Mal'Ganis. Arthas and Muradin went with a group to obtain the runesword Frostmourne, Arthas returned with the bad news that Muradin died, but that they were able to obtain the sword.

After Arthas finally destroyed the dreadlord, the prince along with Froustmourne disappeared into the frozen wastes, leaving the First Legion stranded in Northrend; during that wait, food was scarce and the cold was fierce; In the midst of that, the dwarf Baelgun Flamebeard took command of the original expedition of Muradin's dwarves and they parted ways with the 1st Legion and subsequently venturing into the broken kingdom of Azjol-Nerub.

Captain Falric went to find Arthas but did not return. Arthas slaughtered him and turned him into a death knight. Afterwards, Falric slaughtered Thassarian, and subsequently Arthas wiped out the entire First Legion. Some stayed trapped in the Froustmourne, but many were raised as the undead. Although two soldiers, Orik Trueheart and William Allerton, managed to escape.

Orik and William would later join different forces and return to Northrend to defeat the Lich King Arthas. Orik would return to the forgotten coast, which had been the base of the First Legion in the Third War. Several souls of soldiers from the First Legion who accompanied Arthas in the past were found on the coast, these souls did not seem to know of Arthas' betrayal. Orik enlists the adventurer's help to free the souls from their torment.

In the Reflection chambers you can find Falric and Marwynn (Thassarian would later regain his own will) In addition, in the chambers, you can also find souls of members of the First Legion, in addition to finding mercenary souls. In the chambers, the adventurers defeat Falric and Marwynn, freeing them from their bondage. When the Froustmourne is shattered, presumably all the souls of the First Legion were released.

In the RPG
Oni'jus was one of Prince Arthas' infiltrators before the Third War.

Trivia

 * Some undead models were changed in China due to not being allowed to show bones. Interestingly, the Bone Guards and Risen Soldiers were given the models of Forsaken wearing the tabard of Lordaeron instead, indicating that they may have been previous members of the 1st Legion.
 * In Warcraft III: Reforged, instead of using the name of Expeditionary forces, Arthas's forces use the name of First Legion.

Speculation
It is quite likely that the 1st Legion was also the force which accompanied Arthas in his campaign through plague-ridden Lordaeron as seen in the Warcraft III human campaign. If so, this means that his forces made it to Icecrown after their prince became a death knight.

The mobs in the Halls of Reflection are probably meant to represent yet more members of the 1st Legion, when considering the presence of Shadowy Mercenaries among them.

Torgal and Maxwell were very likely members of the 1st Legion.