From the introduction:
Most of the background for Icefall is contained in The Demonplague campaign’s first two modules, The Frozen Necromancer and The Winter Druids’ Legacy. Icefall picks up where the last adventure left off.
Fel’dryzonna, a comet demon, arrived in the Luna Valley with a small battalion of demons. The comet’s impact melted the Ice Tongue Glacier, throwing the valley into chaos and bringing Fel’dryzonna closer to her goal — waking the mighty plague demon imprisoned beneath the ice and earth, Xancrown.