Boo Lore

The magical abilities of a Boo can be as broad as they want as everyone is able to continue learning new magical abilities.
The only thing limited per Ghost are elemental powers and sub species specific ones (f. e. A bomb boo's smoke will only ever be explosive and can't have healing properties like the one from Dark Boos).

◆Universal to all ghosts are powers like:

-being able to float
-move objects and restrain people's movement
-faze through walls
-making themselves invisible

◆Some additional common magical powers include but are not limited to:

-elemental magic
-healing magic
-miniscule shapeshifting, like changing from a ghost to a more human looking form (only born ghosts or those with gems)
-poisonous smoke
-regenerational magic
-shrinking or enlarging things/people
-hypnosis and mind control
-possession (the power to possess other people or Boos is only granted towards ghosts with powerful magical abilities aka every Boo who's in the possession of some kind of jewel or born boos who trained hard to obtain this power)

While there isn't a set in stone hierarchy of which magical powers are stronger than others since they are only as strong as the wielder is capable of using them, by far the most powerful one is ice magic.
Currently there are three wielders of ice magic but Sebastian is the one with the most concentrated control over it.
Although every Boo if gone rogue could have life threatening magic it is important to remember that there would always be at least one other Boo able to go against them. Even if all else fails, the king has the last laugh and most powerful abilities so he could put an end to this or even break other's magic.

However, ice magic is so strong that it could physically restrain anybody way more efficient than pure strength or other magic would be able to.
It's raw power can not be tamed by any fire related magic especially not Sebastian's.
At it's full power ice magic is able to completely freeze over someone down to the core that if you were to shatter them they would literally break into pieces.
Someone possible wouldn't even notice that they were being attacked until they feel their heart starting to slow down as an ice magic wielder had slowly frozen their heart solid cold.
If an ice magic wielder ever were to loose their mind for what ever reason, only the creator and by that logic The Council would be able to stop them from certain genocide.