Level Up (A5E) DragonCult Culture question

[spartanah]

The wording in this trait is a bit unclear:
Dragon cultists are often
blessed with power from the object of their
devotion so that they may spread their dragon’s
word or further its plans. As a bonus action,
you can cause draconic power to course around
you in a draconic umbra. This draconic umbra
lasts for 1 minute or until you use a bonus action
to end it. Once you have used this trait, you
cannot use it again until after you finish a long
rest. Choose one of the following effects:

Does it mean I chose when I activate the Umbra? or when I build the character?

 

[Titus Divinitus]

It's something you choose at creation that is your permanent Umbra, often tied to the type of dragon the cult worships per the descriptions of each Umbra. You do not get to choose a different Umbra each time you use the ability.