Billowing Mirage (Yellow)

Red Yellow Blue

When this attacks, transform up to 1 ash you control into an Aether Ashwing.

Go again

Rulings

From the Uprising release notes

The transform effect is not targeted. You do not need ash at the time you play Billowing Mirage, only once the effect of Billowing Mirage resolves. This means you can create an ash between playing Billowing Mirage and its effect resolving, and then turn that ash into an Aether Ashwing.
The transform effect is optional. You may choose to transform 0 ash you control.

Keyword notes

Transform - Put the X under this/it/him/her

Create a Y and put the X under it.

Transform is a discrete effect.
To transform subject objects into a specified object, first create the specified object in the arena if it does not yet exist, then put the subjects under it.
To transform objects into a token that does not exist yet, first create the token, then put the objects underneath it.
To transform objects into an existing permanent, just put the objects underneath it.
When used in conjunction with cards that flip (e.g. Invocations), the card is first flipped and becomes a permanent in the arena, then the transform will refer to that permanent as the specified object to transform into (the permanent to put cards under).
The transform fails if there are no transform subjects, or if they can not be put under the specified object for whatever reason.

All keywords

Go again - Go again allows multiple actions to be played in a turn when usually playing an action with use up your action point, and therefore end your turn. A card with go again gives the controller of that card or activated ability 1 action point when it resolves.

When an action with go again resolves, its controller gains 1 action point.
Multiple instances of go again on the same card or activated ability do not stack. If a card or activated ability has multiple instances of go again when it resolves its controller gains 1 action point only.
If an action with go again does not resolve, its controller does not gain 1 action point.
You cannot reveal a card with no to Belittle. The card must have a value to its power that is 3 or less. (0 is a value)