Cryos enables the making of astrals out of heads, by turning them into "the head of <Player> encased in ice".
To get to Cryos, go to the south gate of New Rigel and then go 2 west, 2 north, west, 2 north, west.
Once at Cryos, give 50k gold to him and he will ask for the head. Give him the head and he will give back the head encased in ice, which is a non-decaying astral.
TIP: Bless the head you want to use so that it doesn't decay on your journey to Cryos.