Glacons are hostile mobs that will spawn at any time in all of the snow biomes. If you are not close enough for it to attack you, it will charge at you at twice the speed of a zombie. Between Glacons and Frosts, it is recommended to stay out of Taiga biomes early on if possible. Glacons can also spawn undergound under a Taiga biome.
If you want to fight the Glacons, the best way is to craft yourself a Frost Sword and make yourself some good armor.
The Frost Sword is useful in close combat because it greatly slows down the attacked mob and the Glacon normally runs at a higher speed than a normal Zombie.
The best way to farm Frozen Shards and White Mushrooms is to lure the Glacon into a trap (i.e. 4x3x4 hole) and attack him until he dies.
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