Serperannus does not spawn on her own and requires a Compressed Soil to be summoned.
Serperannus behaves similar to the Eater of Worlds and other worm bosses, burrows through tiles and attempts to hit the player when coming out. Due to the Underground Jungle's vast cavities, she can go a long distance away from the player and perform a charge attack. When she loses some HP, she will turn invulerable and summons in 2 Exile's Underlings to attack the player. She will be vulnerable again when the Underlings are defeated.
If Serperannus is lured out of the Underground Jungle, she will become completely invulnerable to all damage.