In Terraria's Disaster Mode, there is only one way to summon an adult Ghost Sea Dragon: enter a state of chaos. The specific steps are as follows:
Enter the abyss zone from the underground breach.
Click to use the Staff of Chaos in your backpack.
Obtain buffs and put the character in a state of chaos.
After entering the state of chaos, the adult ghost sea monster dragon will be automatically summoned.
The steps above are for reference only. If you still have questions, it is recommended to visit the game's official website.